diff options
Diffstat (limited to 'Documentation/livepatch')
| -rw-r--r-- | Documentation/livepatch/callbacks.txt | 605 | ||||
| -rw-r--r-- | Documentation/livepatch/livepatch.txt | 467 | ||||
| -rw-r--r-- | Documentation/livepatch/module-elf-format.txt | 323 | ||||
| -rw-r--r-- | Documentation/livepatch/shadow-vars.txt | 209 |
4 files changed, 1604 insertions, 0 deletions
diff --git a/Documentation/livepatch/callbacks.txt b/Documentation/livepatch/callbacks.txt new file mode 100644 index 000000000..c9776f48e --- /dev/null +++ b/Documentation/livepatch/callbacks.txt @@ -0,0 +1,605 @@ +====================== +(Un)patching Callbacks +====================== + +Livepatch (un)patch-callbacks provide a mechanism for livepatch modules +to execute callback functions when a kernel object is (un)patched. They +can be considered a "power feature" that extends livepatching abilities +to include: + + - Safe updates to global data + + - "Patches" to init and probe functions + + - Patching otherwise unpatchable code (i.e. assembly) + +In most cases, (un)patch callbacks will need to be used in conjunction +with memory barriers and kernel synchronization primitives, like +mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues. + +Callbacks differ from existing kernel facilities: + + - Module init/exit code doesn't run when disabling and re-enabling a + patch. + + - A module notifier can't stop a to-be-patched module from loading. + +Callbacks are part of the klp_object structure and their implementation +is specific to that klp_object. Other livepatch objects may or may not +be patched, irrespective of the target klp_object's current state. + +Callbacks can be registered for the following livepatch actions: + + * Pre-patch - before a klp_object is patched + + * Post-patch - after a klp_object has been patched and is active + across all tasks + + * Pre-unpatch - before a klp_object is unpatched (ie, patched code is + active), used to clean up post-patch callback + resources + + * Post-unpatch - after a klp_object has been patched, all code has + been restored and no tasks are running patched code, + used to cleanup pre-patch callback resources + +Each callback is optional, omitting one does not preclude specifying any +other. However, the livepatching core executes the handlers in +symmetry: pre-patch callbacks have a post-unpatch counterpart and +post-patch callbacks have a pre-unpatch counterpart. An unpatch +callback will only be executed if its corresponding patch callback was +executed. Typical use cases pair a patch handler that acquires and +configures resources with an unpatch handler tears down and releases +those same resources. + +A callback is only executed if its host klp_object is loaded. For +in-kernel vmlinux targets, this means that callbacks will always execute +when a livepatch is enabled/disabled. For patch target kernel modules, +callbacks will only execute if the target module is loaded. When a +module target is (un)loaded, its callbacks will execute only if the +livepatch module is enabled. + +The pre-patch callback, if specified, is expected to return a status +code (0 for success, -ERRNO on error). An error status code indicates +to the livepatching core that patching of the current klp_object is not +safe and to stop the current patching request. (When no pre-patch +callback is provided, the transition is assumed to be safe.) If a +pre-patch callback returns failure, the kernel's module loader will: + + - Refuse to load a livepatch, if the livepatch is loaded after + targeted code. + + or: + + - Refuse to load a module, if the livepatch was already successfully + loaded. + +No post-patch, pre-unpatch, or post-unpatch callbacks will be executed +for a given klp_object if the object failed to patch, due to a failed +pre_patch callback or for any other reason. + +If a patch transition is reversed, no pre-unpatch handlers will be run +(this follows the previously mentioned symmetry -- pre-unpatch callbacks +will only occur if their corresponding post-patch callback executed). + +If the object did successfully patch, but the patch transition never +started for some reason (e.g., if another object failed to patch), +only the post-unpatch callback will be called. + + +Example Use-cases +================= + +Update global data +------------------ + +A pre-patch callback can be useful to update a global variable. For +example, 75ff39ccc1bd ("tcp: make challenge acks less predictable") +changes a global sysctl, as well as patches the tcp_send_challenge_ack() +function. + +In this case, if we're being super paranoid, it might make sense to +patch the data *after* patching is complete with a post-patch callback, +so that tcp_send_challenge_ack() could first be changed to read +sysctl_tcp_challenge_ack_limit with READ_ONCE. + + +Support __init and probe function patches +----------------------------------------- + +Although __init and probe functions are not directly livepatch-able, it +may be possible to implement similar updates via pre/post-patch +callbacks. + +48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST") change the way that +virtnet_probe() initialized its driver's net_device features. A +pre/post-patch callback could iterate over all such devices, making a +similar change to their hw_features value. (Client functions of the +value may need to be updated accordingly.) + + +Test cases +========== + +What follows is not an exhaustive test suite of every possible livepatch +pre/post-(un)patch combination, but a selection that demonstrates a few +important concepts. Each test case uses the kernel modules located in +the samples/livepatch/ and assumes that no livepatches are loaded at the +beginning of the test. + + +Test 1 +------ + +Test a combination of loading a kernel module and a livepatch that +patches a function in the first module. (Un)load the target module +before the livepatch module: + +- load target module +- load livepatch +- disable livepatch +- unload target module +- unload livepatch + +First load a target module: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 34.475708] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +On livepatch enable, before the livepatch transition starts, pre-patch +callbacks are executed for vmlinux and livepatch_callbacks_mod (those +klp_objects currently loaded). After klp_objects are patched according +to the klp_patch, their post-patch callbacks run and the transition +completes: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 36.503719] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 36.504213] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 36.504238] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 36.504721] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 36.505849] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 37.727133] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 37.727232] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 37.727860] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 37.728792] livepatch: 'livepatch_callbacks_demo': patching complete + +Similarly, on livepatch disable, pre-patch callbacks run before the +unpatching transition starts. klp_objects are reverted, post-patch +callbacks execute and the transition completes: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 38.510209] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 38.510234] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 38.510982] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 38.512209] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 39.711132] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 39.711210] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 39.711779] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 39.712735] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 42.534183] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + + +Test 2 +------ + +This test is similar to the previous test, but (un)load the livepatch +module before the target kernel module. This tests the livepatch core's +module_coming handler: + +- load livepatch +- load target module +- disable livepatch +- unload livepatch +- unload target module + + +On livepatch enable, only pre/post-patch callbacks are executed for +currently loaded klp_objects, in this case, vmlinux: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 44.553328] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 44.553997] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 44.554049] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 44.554845] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 45.727128] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 45.727212] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 45.727961] livepatch: 'livepatch_callbacks_demo': patching complete + +When a targeted module is subsequently loaded, only its pre/post-patch +callbacks are executed: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 46.560845] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 46.561988] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 46.563452] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 46.565495] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +On livepatch disable, all currently loaded klp_objects' (vmlinux and +livepatch_callbacks_mod) pre/post-unpatch callbacks are executed: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 48.568885] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 48.568910] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 48.569441] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 48.570502] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 49.759091] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 49.759171] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 49.759742] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 49.760690] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 52.592283] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + + +Test 3 +------ + +Test loading the livepatch after a targeted kernel module, then unload +the kernel module before disabling the livepatch. This tests the +livepatch core's module_going handler: + +- load target module +- load livepatch +- unload target module +- disable livepatch +- unload livepatch + +First load a target module, then the livepatch: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 54.607948] livepatch_callbacks_mod: livepatch_callbacks_mod_init + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 56.613919] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 56.614411] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 56.614436] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 56.614818] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 56.615656] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 57.759070] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 57.759147] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 57.759621] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 57.760307] livepatch: 'livepatch_callbacks_demo': patching complete + +When a target module is unloaded, the livepatch is only reverted from +that klp_object (livepatch_callbacks_mod). As such, only its pre and +post-unpatch callbacks are executed when this occurs: + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 58.623409] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + [ 58.623903] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + [ 58.624658] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod' + [ 58.625305] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + +When the livepatch is disabled, pre and post-unpatch callbacks are run +for the remaining klp_object, vmlinux: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 60.638420] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 60.638444] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 60.638996] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 61.727088] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 61.727165] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 61.727985] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + + +Test 4 +------ + +This test is similar to the previous test, however the livepatch is +loaded first. This tests the livepatch core's module_coming and +module_going handlers: + +- load livepatch +- load target module +- unload target module +- disable livepatch +- unload livepatch + +First load the livepatch: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 64.661552] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 64.662147] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 64.662175] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 64.662850] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 65.695056] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 65.695147] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 65.695561] livepatch: 'livepatch_callbacks_demo': patching complete + +When a targeted kernel module is subsequently loaded, only its +pre/post-patch callbacks are executed: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 66.669196] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 66.669882] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 66.670744] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 66.672873] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +When the target module is unloaded, the livepatch is only reverted from +the livepatch_callbacks_mod klp_object. As such, only pre and +post-unpatch callbacks are executed when this occurs: + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 68.680065] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + [ 68.680688] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + [ 68.681452] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod' + [ 68.682094] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 70.689225] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 70.689256] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 70.689882] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 71.711080] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 71.711481] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 71.711988] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + + +Test 5 +------ + +A simple test of loading a livepatch without one of its patch target +klp_objects ever loaded (livepatch_callbacks_mod): + +- load livepatch +- disable livepatch +- unload livepatch + +Load the livepatch: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 74.711081] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 74.711595] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 74.711639] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 74.712272] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 75.743137] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 75.743219] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 75.743867] livepatch: 'livepatch_callbacks_demo': patching complete + +As expected, only pre/post-(un)patch handlers are executed for vmlinux: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 76.716254] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 76.716278] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 76.716666] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 77.727089] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 77.727194] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 77.727907] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + + +Test 6 +------ + +Test a scenario where a vmlinux pre-patch callback returns a non-zero +status (ie, failure): + +- load target module +- load livepatch -ENODEV +- unload target module + +First load a target module: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 80.740520] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +Load the livepatch module, setting its 'pre_patch_ret' value to -19 +(-ENODEV). When its vmlinux pre-patch callback executed, this status +code will propagate back to the module-loading subsystem. The result is +that the insmod command refuses to load the livepatch module: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko pre_patch_ret=-19 + [ 82.747326] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 82.747743] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 82.747767] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 82.748237] livepatch: pre-patch callback failed for object 'vmlinux' + [ 82.748637] livepatch: failed to enable patch 'livepatch_callbacks_demo' + [ 82.749059] livepatch: 'livepatch_callbacks_demo': canceling transition, going to unpatch + [ 82.749060] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 82.749868] livepatch: 'livepatch_callbacks_demo': unpatching complete + [ 82.765809] insmod: ERROR: could not insert module samples/livepatch/livepatch-callbacks-demo.ko: No such device + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 84.774238] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + + +Test 7 +------ + +Similar to the previous test, setup a livepatch such that its vmlinux +pre-patch callback returns success. However, when a targeted kernel +module is later loaded, have the livepatch return a failing status code: + +- load livepatch +- setup -ENODEV +- load target module +- disable livepatch +- unload livepatch + +Load the livepatch, notice vmlinux pre-patch callback succeeds: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 86.787845] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 86.788325] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 86.788427] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 86.788821] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 87.711069] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 87.711143] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 87.711886] livepatch: 'livepatch_callbacks_demo': patching complete + +Set a trap so subsequent pre-patch callbacks to this livepatch will +return -ENODEV: + + % echo -19 > /sys/module/livepatch_callbacks_demo/parameters/pre_patch_ret + +The livepatch pre-patch callback for subsequently loaded target modules +will return failure, so the module loader refuses to load the kernel +module. Notice that no post-patch or pre/post-unpatch callbacks are +executed for this klp_object: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 90.796976] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 90.797834] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 90.798900] livepatch: pre-patch callback failed for object 'livepatch_callbacks_mod' + [ 90.799652] livepatch: patch 'livepatch_callbacks_demo' failed for module 'livepatch_callbacks_mod', refusing to load module 'livepatch_callbacks_mod' + [ 90.819737] insmod: ERROR: could not insert module samples/livepatch/livepatch-callbacks-mod.ko: No such device + +However, pre/post-unpatch callbacks run for the vmlinux klp_object: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 92.823547] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 92.823573] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 92.824331] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 93.727128] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 93.727327] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 93.727861] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + + +Test 8 +------ + +Test loading multiple targeted kernel modules. This test-case is +mainly for comparing with the next test-case. + +- load busy target module (0s sleep), +- load livepatch +- load target module +- unload target module +- disable livepatch +- unload livepatch +- unload busy target module + + +Load a target "busy" kernel module which kicks off a worker function +that immediately exits: + + % insmod samples/livepatch/livepatch-callbacks-busymod.ko sleep_secs=0 + [ 96.910107] livepatch_callbacks_busymod: livepatch_callbacks_mod_init + [ 96.910600] livepatch_callbacks_busymod: busymod_work_func, sleeping 0 seconds ... + [ 96.913024] livepatch_callbacks_busymod: busymod_work_func exit + +Proceed with loading the livepatch and another ordinary target module, +notice that the post-patch callbacks are executed and the transition +completes quickly: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 98.917892] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 98.918426] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 98.918453] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 98.918955] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 98.923835] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 99.743104] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 99.743156] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 99.743679] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 99.744616] livepatch: 'livepatch_callbacks_demo': patching complete + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 100.930955] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 100.931668] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 100.932645] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 100.934125] livepatch_callbacks_mod: livepatch_callbacks_mod_init + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 102.942805] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + [ 102.943640] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + [ 102.944585] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod' + [ 102.945455] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 104.953815] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 104.953838] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 104.954431] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 104.955426] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 106.719073] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 106.722633] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 106.723282] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 106.724279] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + % rmmod samples/livepatch/livepatch-callbacks-busymod.ko + [ 108.975660] livepatch_callbacks_busymod: livepatch_callbacks_mod_exit + + +Test 9 +------ + +A similar test as the previous one, but force the "busy" kernel module +to do longer work. + +The livepatching core will refuse to patch a task that is currently +executing a to-be-patched function -- the consistency model stalls the +current patch transition until this safety-check is met. Test a +scenario where one of a livepatch's target klp_objects sits on such a +function for a long time. Meanwhile, load and unload other target +kernel modules while the livepatch transition is in progress. + +- load busy target module (30s sleep) +- load livepatch +- load target module +- unload target module +- disable livepatch +- unload livepatch +- unload busy target module + + +Load the "busy" kernel module, this time make it do 30 seconds worth of +work: + + % insmod samples/livepatch/livepatch-callbacks-busymod.ko sleep_secs=30 + [ 110.993362] livepatch_callbacks_busymod: livepatch_callbacks_mod_init + [ 110.994059] livepatch_callbacks_busymod: busymod_work_func, sleeping 30 seconds ... + +Meanwhile, the livepatch is loaded. Notice that the patch transition +does not complete as the targeted "busy" module is sitting on a +to-be-patched function: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 113.000309] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 113.000764] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 113.000791] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 113.001289] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 113.005208] livepatch: 'livepatch_callbacks_demo': starting patching transition + +Load a second target module (this one is an ordinary idle kernel +module). Note that *no* post-patch callbacks will be executed while the +livepatch is still in transition: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 115.012740] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 115.013406] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 115.015315] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +Request an unload of the simple kernel module. The patch is still +transitioning, so its pre-unpatch callbacks are skipped: + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 117.022626] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + [ 117.023376] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod' + [ 117.024533] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + +Finally the livepatch is disabled. Since none of the patch's +klp_object's post-patch callbacks executed, the remaining klp_object's +pre-unpatch callbacks are skipped: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 119.035408] livepatch: 'livepatch_callbacks_demo': reversing transition from patching to unpatching + [ 119.035485] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 119.711166] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 119.714179] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 119.714653] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 119.715437] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + % rmmod samples/livepatch/livepatch-callbacks-busymod.ko + [ 141.279111] livepatch_callbacks_busymod: busymod_work_func exit + [ 141.279760] livepatch_callbacks_busymod: livepatch_callbacks_mod_exit diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt new file mode 100644 index 000000000..2d7ed09db --- /dev/null +++ b/Documentation/livepatch/livepatch.txt @@ -0,0 +1,467 @@ +========= +Livepatch +========= + +This document outlines basic information about kernel livepatching. + +Table of Contents: + +1. Motivation +2. Kprobes, Ftrace, Livepatching +3. Consistency model +4. Livepatch module + 4.1. New functions + 4.2. Metadata + 4.3. Livepatch module handling +5. Livepatch life-cycle + 5.1. Registration + 5.2. Enabling + 5.3. Disabling + 5.4. Unregistration +6. Sysfs +7. Limitations + + +1. Motivation +============= + +There are many situations where users are reluctant to reboot a system. It may +be because their system is performing complex scientific computations or under +heavy load during peak usage. In addition to keeping systems up and running, +users want to also have a stable and secure system. Livepatching gives users +both by allowing for function calls to be redirected; thus, fixing critical +functions without a system reboot. + + +2. Kprobes, Ftrace, Livepatching +================================ + +There are multiple mechanisms in the Linux kernel that are directly related +to redirection of code execution; namely: kernel probes, function tracing, +and livepatching: + + + The kernel probes are the most generic. The code can be redirected by + putting a breakpoint instruction instead of any instruction. + + + The function tracer calls the code from a predefined location that is + close to the function entry point. This location is generated by the + compiler using the '-pg' gcc option. + + + Livepatching typically needs to redirect the code at the very beginning + of the function entry before the function parameters or the stack + are in any way modified. + +All three approaches need to modify the existing code at runtime. Therefore +they need to be aware of each other and not step over each other's toes. +Most of these problems are solved by using the dynamic ftrace framework as +a base. A Kprobe is registered as a ftrace handler when the function entry +is probed, see CONFIG_KPROBES_ON_FTRACE. Also an alternative function from +a live patch is called with the help of a custom ftrace handler. But there are +some limitations, see below. + + +3. Consistency model +==================== + +Functions are there for a reason. They take some input parameters, get or +release locks, read, process, and even write some data in a defined way, +have return values. In other words, each function has a defined semantic. + +Many fixes do not change the semantic of the modified functions. For +example, they add a NULL pointer or a boundary check, fix a race by adding +a missing memory barrier, or add some locking around a critical section. +Most of these changes are self contained and the function presents itself +the same way to the rest of the system. In this case, the functions might +be updated independently one by one. + +But there are more complex fixes. For example, a patch might change +ordering of locking in multiple functions at the same time. Or a patch +might exchange meaning of some temporary structures and update +all the relevant functions. In this case, the affected unit +(thread, whole kernel) need to start using all new versions of +the functions at the same time. Also the switch must happen only +when it is safe to do so, e.g. when the affected locks are released +or no data are stored in the modified structures at the moment. + +The theory about how to apply functions a safe way is rather complex. +The aim is to define a so-called consistency model. It attempts to define +conditions when the new implementation could be used so that the system +stays consistent. + +Livepatch has a consistency model which is a hybrid of kGraft and +kpatch: it uses kGraft's per-task consistency and syscall barrier +switching combined with kpatch's stack trace switching. There are also +a number of fallback options which make it quite flexible. + +Patches are applied on a per-task basis, when the task is deemed safe to +switch over. When a patch is enabled, livepatch enters into a +transition state where tasks are converging to the patched state. +Usually this transition state can complete in a few seconds. The same +sequence occurs when a patch is disabled, except the tasks converge from +the patched state to the unpatched state. + +An interrupt handler inherits the patched state of the task it +interrupts. The same is true for forked tasks: the child inherits the +patched state of the parent. + +Livepatch uses several complementary approaches to determine when it's +safe to patch tasks: + +1. The first and most effective approach is stack checking of sleeping + tasks. If no affected functions are on the stack of a given task, + the task is patched. In most cases this will patch most or all of + the tasks on the first try. Otherwise it'll keep trying + periodically. This option is only available if the architecture has + reliable stacks (HAVE_RELIABLE_STACKTRACE). + +2. The second approach, if needed, is kernel exit switching. A + task is switched when it returns to user space from a system call, a + user space IRQ, or a signal. It's useful in the following cases: + + a) Patching I/O-bound user tasks which are sleeping on an affected + function. In this case you have to send SIGSTOP and SIGCONT to + force it to exit the kernel and be patched. + b) Patching CPU-bound user tasks. If the task is highly CPU-bound + then it will get patched the next time it gets interrupted by an + IRQ. + +3. For idle "swapper" tasks, since they don't ever exit the kernel, they + instead have a klp_update_patch_state() call in the idle loop which + allows them to be patched before the CPU enters the idle state. + + (Note there's not yet such an approach for kthreads.) + +Architectures which don't have HAVE_RELIABLE_STACKTRACE solely rely on +the second approach. It's highly likely that some tasks may still be +running with an old version of the function, until that function +returns. In this case you would have to signal the tasks. This +especially applies to kthreads. They may not be woken up and would need +to be forced. See below for more information. + +Unless we can come up with another way to patch kthreads, architectures +without HAVE_RELIABLE_STACKTRACE are not considered fully supported by +the kernel livepatching. + +The /sys/kernel/livepatch/<patch>/transition file shows whether a patch +is in transition. Only a single patch (the topmost patch on the stack) +can be in transition at a given time. A patch can remain in transition +indefinitely, if any of the tasks are stuck in the initial patch state. + +A transition can be reversed and effectively canceled by writing the +opposite value to the /sys/kernel/livepatch/<patch>/enabled file while +the transition is in progress. Then all the tasks will attempt to +converge back to the original patch state. + +There's also a /proc/<pid>/patch_state file which can be used to +determine which tasks are blocking completion of a patching operation. +If a patch is in transition, this file shows 0 to indicate the task is +unpatched and 1 to indicate it's patched. Otherwise, if no patch is in +transition, it shows -1. Any tasks which are blocking the transition +can be signaled with SIGSTOP and SIGCONT to force them to change their +patched state. This may be harmful to the system though. +/sys/kernel/livepatch/<patch>/signal attribute provides a better alternative. +Writing 1 to the attribute sends a fake signal to all remaining blocking +tasks. No proper signal is actually delivered (there is no data in signal +pending structures). Tasks are interrupted or woken up, and forced to change +their patched state. + +Administrator can also affect a transition through +/sys/kernel/livepatch/<patch>/force attribute. Writing 1 there clears +TIF_PATCH_PENDING flag of all tasks and thus forces the tasks to the patched +state. Important note! The force attribute is intended for cases when the +transition gets stuck for a long time because of a blocking task. Administrator +is expected to collect all necessary data (namely stack traces of such blocking +tasks) and request a clearance from a patch distributor to force the transition. +Unauthorized usage may cause harm to the system. It depends on the nature of the +patch, which functions are (un)patched, and which functions the blocking tasks +are sleeping in (/proc/<pid>/stack may help here). Removal (rmmod) of patch +modules is permanently disabled when the force feature is used. It cannot be +guaranteed there is no task sleeping in such module. It implies unbounded +reference count if a patch module is disabled and enabled in a loop. + +Moreover, the usage of force may also affect future applications of live +patches and cause even more harm to the system. Administrator should first +consider to simply cancel a transition (see above). If force is used, reboot +should be planned and no more live patches applied. + +3.1 Adding consistency model support to new architectures +--------------------------------------------------------- + +For adding consistency model support to new architectures, there are a +few options: + +1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and + for non-DWARF unwinders, also making sure there's a way for the stack + tracing code to detect interrupts on the stack. + +2) Alternatively, ensure that every kthread has a call to + klp_update_patch_state() in a safe location. Kthreads are typically + in an infinite loop which does some action repeatedly. The safe + location to switch the kthread's patch state would be at a designated + point in the loop where there are no locks taken and all data + structures are in a well-defined state. + + The location is clear when using workqueues or the kthread worker + API. These kthreads process independent actions in a generic loop. + + It's much more complicated with kthreads which have a custom loop. + There the safe location must be carefully selected on a case-by-case + basis. + + In that case, arches without HAVE_RELIABLE_STACKTRACE would still be + able to use the non-stack-checking parts of the consistency model: + + a) patching user tasks when they cross the kernel/user space + boundary; and + + b) patching kthreads and idle tasks at their designated patch points. + + This option isn't as good as option 1 because it requires signaling + user tasks and waking kthreads to patch them. But it could still be + a good backup option for those architectures which don't have + reliable stack traces yet. + + +4. Livepatch module +=================== + +Livepatches are distributed using kernel modules, see +samples/livepatch/livepatch-sample.c. + +The module includes a new implementation of functions that we want +to replace. In addition, it defines some structures describing the +relation between the original and the new implementation. Then there +is code that makes the kernel start using the new code when the livepatch +module is loaded. Also there is code that cleans up before the +livepatch module is removed. All this is explained in more details in +the next sections. + + +4.1. New functions +------------------ + +New versions of functions are typically just copied from the original +sources. A good practice is to add a prefix to the names so that they +can be distinguished from the original ones, e.g. in a backtrace. Also +they can be declared as static because they are not called directly +and do not need the global visibility. + +The patch contains only functions that are really modified. But they +might want to access functions or data from the original source file +that may only be locally accessible. This can be solved by a special +relocation section in the generated livepatch module, see +Documentation/livepatch/module-elf-format.txt for more details. + + +4.2. Metadata +------------- + +The patch is described by several structures that split the information +into three levels: + + + struct klp_func is defined for each patched function. It describes + the relation between the original and the new implementation of a + particular function. + + The structure includes the name, as a string, of the original function. + The function address is found via kallsyms at runtime. + + Then it includes the address of the new function. It is defined + directly by assigning the function pointer. Note that the new + function is typically defined in the same source file. + + As an optional parameter, the symbol position in the kallsyms database can + be used to disambiguate functions of the same name. This is not the + absolute position in the database, but rather the order it has been found + only for a particular object ( vmlinux or a kernel module ). Note that + kallsyms allows for searching symbols according to the object name. + + + struct klp_object defines an array of patched functions (struct + klp_func) in the same object. Where the object is either vmlinux + (NULL) or a module name. + + The structure helps to group and handle functions for each object + together. Note that patched modules might be loaded later than + the patch itself and the relevant functions might be patched + only when they are available. + + + + struct klp_patch defines an array of patched objects (struct + klp_object). + + This structure handles all patched functions consistently and eventually, + synchronously. The whole patch is applied only when all patched + symbols are found. The only exception are symbols from objects + (kernel modules) that have not been loaded yet. + + For more details on how the patch is applied on a per-task basis, + see the "Consistency model" section. + + +4.3. Livepatch module handling +------------------------------ + +The usual behavior is that the new functions will get used when +the livepatch module is loaded. For this, the module init() function +has to register the patch (struct klp_patch) and enable it. See the +section "Livepatch life-cycle" below for more details about these +two operations. + +Module removal is only safe when there are no users of the underlying +functions. This is the reason why the force feature permanently disables +the removal. The forced tasks entered the functions but we cannot say +that they returned back. Therefore it cannot be decided when the +livepatch module can be safely removed. When the system is successfully +transitioned to a new patch state (patched/unpatched) without being +forced it is guaranteed that no task sleeps or runs in the old code. + + +5. Livepatch life-cycle +======================= + +Livepatching defines four basic operations that define the life cycle of each +live patch: registration, enabling, disabling and unregistration. There are +several reasons why it is done this way. + +First, the patch is applied only when all patched symbols for already +loaded objects are found. The error handling is much easier if this +check is done before particular functions get redirected. + +Second, it might take some time until the entire system is migrated with +the hybrid consistency model being used. The patch revert might block +the livepatch module removal for too long. Therefore it is useful to +revert the patch using a separate operation that might be called +explicitly. But it does not make sense to remove all information until +the livepatch module is really removed. + + +5.1. Registration +----------------- + +Each patch first has to be registered using klp_register_patch(). This makes +the patch known to the livepatch framework. Also it does some preliminary +computing and checks. + +In particular, the patch is added into the list of known patches. The +addresses of the patched functions are found according to their names. +The special relocations, mentioned in the section "New functions", are +applied. The relevant entries are created under +/sys/kernel/livepatch/<name>. The patch is rejected when any operation +fails. + + +5.2. Enabling +------------- + +Registered patches might be enabled either by calling klp_enable_patch() or +by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will +start using the new implementation of the patched functions at this stage. + +When a patch is enabled, livepatch enters into a transition state where +tasks are converging to the patched state. This is indicated by a value +of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks have +been patched, the 'transition' value changes to '0'. For more +information about this process, see the "Consistency model" section. + +If an original function is patched for the first time, a function +specific struct klp_ops is created and an universal ftrace handler is +registered. + +Functions might be patched multiple times. The ftrace handler is registered +only once for the given function. Further patches just add an entry to the +list (see field `func_stack`) of the struct klp_ops. The last added +entry is chosen by the ftrace handler and becomes the active function +replacement. + +Note that the patches might be enabled in a different order than they were +registered. + + +5.3. Disabling +-------------- + +Enabled patches might get disabled either by calling klp_disable_patch() or +by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage +either the code from the previously enabled patch or even the original +code gets used. + +When a patch is disabled, livepatch enters into a transition state where +tasks are converging to the unpatched state. This is indicated by a +value of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks +have been unpatched, the 'transition' value changes to '0'. For more +information about this process, see the "Consistency model" section. + +Here all the functions (struct klp_func) associated with the to-be-disabled +patch are removed from the corresponding struct klp_ops. The ftrace handler +is unregistered and the struct klp_ops is freed when the func_stack list +becomes empty. + +Patches must be disabled in exactly the reverse order in which they were +enabled. It makes the problem and the implementation much easier. + + +5.4. Unregistration +------------------- + +Disabled patches might be unregistered by calling klp_unregister_patch(). +This can be done only when the patch is disabled and the code is no longer +used. It must be called before the livepatch module gets unloaded. + +At this stage, all the relevant sys-fs entries are removed and the patch +is removed from the list of known patches. + + +6. Sysfs +======== + +Information about the registered patches can be found under +/sys/kernel/livepatch. The patches could be enabled and disabled +by writing there. + +/sys/kernel/livepatch/<patch>/signal and /sys/kernel/livepatch/<patch>/force +attributes allow administrator to affect a patching operation. + +See Documentation/ABI/testing/sysfs-kernel-livepatch for more details. + + +7. Limitations +============== + +The current Livepatch implementation has several limitations: + + + Only functions that can be traced could be patched. + + Livepatch is based on the dynamic ftrace. In particular, functions + implementing ftrace or the livepatch ftrace handler could not be + patched. Otherwise, the code would end up in an infinite loop. A + potential mistake is prevented by marking the problematic functions + by "notrace". + + + + + Livepatch works reliably only when the dynamic ftrace is located at + the very beginning of the function. + + The function need to be redirected before the stack or the function + parameters are modified in any way. For example, livepatch requires + using -fentry gcc compiler option on x86_64. + + One exception is the PPC port. It uses relative addressing and TOC. + Each function has to handle TOC and save LR before it could call + the ftrace handler. This operation has to be reverted on return. + Fortunately, the generic ftrace code has the same problem and all + this is handled on the ftrace level. + + + + Kretprobes using the ftrace framework conflict with the patched + functions. + + Both kretprobes and livepatches use a ftrace handler that modifies + the return address. The first user wins. Either the probe or the patch + is rejected when the handler is already in use by the other. + + + + Kprobes in the original function are ignored when the code is + redirected to the new implementation. + + There is a work in progress to add warnings about this situation. diff --git a/Documentation/livepatch/module-elf-format.txt b/Documentation/livepatch/module-elf-format.txt new file mode 100644 index 000000000..f21a5289a --- /dev/null +++ b/Documentation/livepatch/module-elf-format.txt @@ -0,0 +1,323 @@ +=========================== +Livepatch module Elf format +=========================== + +This document outlines the Elf format requirements that livepatch modules must follow. + +----------------- +Table of Contents +----------------- +0. Background and motivation +1. Livepatch modinfo field +2. Livepatch relocation sections + 2.1 What are livepatch relocation sections? + 2.2 Livepatch relocation section format + 2.2.1 Required flags + 2.2.2 Required name format + 2.2.3 Example livepatch relocation section names + 2.2.4 Example `readelf --sections` output + 2.2.5 Example `readelf --relocs` output +3. Livepatch symbols + 3.1 What are livepatch symbols? + 3.2 A livepatch module's symbol table + 3.3 Livepatch symbol format + 3.3.1 Required flags + 3.3.2 Required name format + 3.3.3 Example livepatch symbol names + 3.3.4 Example `readelf --symbols` output +4. Architecture-specific sections +5. Symbol table and Elf section access + +---------------------------- +0. Background and motivation +---------------------------- + +Formerly, livepatch required separate architecture-specific code to write +relocations. However, arch-specific code to write relocations already +exists in the module loader, so this former approach produced redundant +code. So, instead of duplicating code and re-implementing what the module +loader can already do, livepatch leverages existing code in the module +loader to perform the all the arch-specific relocation work. Specifically, +livepatch reuses the apply_relocate_add() function in the module loader to +write relocations. The patch module Elf format described in this document +enables livepatch to be able to do this. The hope is that this will make +livepatch more easily portable to other architectures and reduce the amount +of arch-specific code required to port livepatch to a particular +architecture. + +Since apply_relocate_add() requires access to a module's section header +table, symbol table, and relocation section indices, Elf information is +preserved for livepatch modules (see section 5). Livepatch manages its own +relocation sections and symbols, which are described in this document. The +Elf constants used to mark livepatch symbols and relocation sections were +selected from OS-specific ranges according to the definitions from glibc. + +0.1 Why does livepatch need to write its own relocations? +--------------------------------------------------------- +A typical livepatch module contains patched versions of functions that can +reference non-exported global symbols and non-included local symbols. +Relocations referencing these types of symbols cannot be left in as-is +since the kernel module loader cannot resolve them and will therefore +reject the livepatch module. Furthermore, we cannot apply relocations that +affect modules not yet loaded at patch module load time (e.g. a patch to a +driver that is not loaded). Formerly, livepatch solved this problem by +embedding special "dynrela" (dynamic rela) sections in the resulting patch +module Elf output. Using these dynrela sections, livepatch could resolve +symbols while taking into account its scope and what module the symbol +belongs to, and then manually apply the dynamic relocations. However this +approach required livepatch to supply arch-specific code in order to write +these relocations. In the new format, livepatch manages its own SHT_RELA +relocation sections in place of dynrela sections, and the symbols that the +relas reference are special livepatch symbols (see section 2 and 3). The +arch-specific livepatch relocation code is replaced by a call to +apply_relocate_add(). + +================================ +PATCH MODULE FORMAT REQUIREMENTS +================================ + +-------------------------- +1. Livepatch modinfo field +-------------------------- + +Livepatch modules are required to have the "livepatch" modinfo attribute. +See the sample livepatch module in samples/livepatch/ for how this is done. + +Livepatch modules can be identified by users by using the 'modinfo' command +and looking for the presence of the "livepatch" field. This field is also +used by the kernel module loader to identify livepatch modules. + +Example modinfo output: +----------------------- +% modinfo livepatch-meminfo.ko +filename: livepatch-meminfo.ko +livepatch: Y +license: GPL +depends: +vermagic: 4.3.0+ SMP mod_unload + +-------------------------------- +2. Livepatch relocation sections +-------------------------------- + +------------------------------------------- +2.1 What are livepatch relocation sections? +------------------------------------------- +A livepatch module manages its own Elf relocation sections to apply +relocations to modules as well as to the kernel (vmlinux) at the +appropriate time. For example, if a patch module patches a driver that is +not currently loaded, livepatch will apply the corresponding livepatch +relocation section(s) to the driver once it loads. + +Each "object" (e.g. vmlinux, or a module) within a patch module may have +multiple livepatch relocation sections associated with it (e.g. patches to +multiple functions within the same object). There is a 1-1 correspondence +between a livepatch relocation section and the target section (usually the +text section of a function) to which the relocation(s) apply. It is +also possible for a livepatch module to have no livepatch relocation +sections, as in the case of the sample livepatch module (see +samples/livepatch). + +Since Elf information is preserved for livepatch modules (see Section 5), a +livepatch relocation section can be applied simply by passing in the +appropriate section index to apply_relocate_add(), which then uses it to +access the relocation section and apply the relocations. + +Every symbol referenced by a rela in a livepatch relocation section is a +livepatch symbol. These must be resolved before livepatch can call +apply_relocate_add(). See Section 3 for more information. + +--------------------------------------- +2.2 Livepatch relocation section format +--------------------------------------- + +2.2.1 Required flags +-------------------- +Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH +section flag. See include/uapi/linux/elf.h for the definition. The module +loader recognizes this flag and will avoid applying those relocation sections +at patch module load time. These sections must also be marked with SHF_ALLOC, +so that the module loader doesn't discard them on module load (i.e. they will +be copied into memory along with the other SHF_ALLOC sections). + +2.2.2 Required name format +-------------------------- +The name of a livepatch relocation section must conform to the following format: + +.klp.rela.objname.section_name +^ ^^ ^ ^ ^ +|________||_____| |__________| + [A] [B] [C] + +[A] The relocation section name is prefixed with the string ".klp.rela." +[B] The name of the object (i.e. "vmlinux" or name of module) to + which the relocation section belongs follows immediately after the prefix. +[C] The actual name of the section to which this relocation section applies. + +2.2.3 Example livepatch relocation section names: +------------------------------------------------- +.klp.rela.ext4.text.ext4_attr_store +.klp.rela.vmlinux.text.cmdline_proc_show + +2.2.4 Example `readelf --sections` output for a patch +module that patches vmlinux and modules 9p, btrfs, ext4: +-------------------------------------------------------- + Section Headers: + [Nr] Name Type Address Off Size ES Flg Lk Inf Al + [ snip ] + [29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8 + [30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8 + [ snip ] + [34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8 + [35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8 + [36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8 + [37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8 + [ snip ] ^ ^ + | | + [*] [*] +[*] Livepatch relocation sections are SHT_RELA sections but with a few special +characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will +not be discarded when the module is loaded into memory, as well as with the +SHF_RELA_LIVEPATCH flag ("o" - for OS-specific). + +2.2.5 Example `readelf --relocs` output for a patch module: +----------------------------------------------------------- +Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries: + Offset Info Type Symbol's Value Symbol's Name + Addend +000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4 +0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0 +0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4 +000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4 +[ snip ] ^ + | + [*] +[*] Every symbol referenced by a relocation is a livepatch symbol. + +-------------------- +3. Livepatch symbols +-------------------- + +------------------------------- +3.1 What are livepatch symbols? +------------------------------- +Livepatch symbols are symbols referred to by livepatch relocation sections. +These are symbols accessed from new versions of functions for patched +objects, whose addresses cannot be resolved by the module loader (because +they are local or unexported global syms). Since the module loader only +resolves exported syms, and not every symbol referenced by the new patched +functions is exported, livepatch symbols were introduced. They are used +also in cases where we cannot immediately know the address of a symbol when +a patch module loads. For example, this is the case when livepatch patches +a module that is not loaded yet. In this case, the relevant livepatch +symbols are resolved simply when the target module loads. In any case, for +any livepatch relocation section, all livepatch symbols referenced by that +section must be resolved before livepatch can call apply_relocate_add() for +that reloc section. + +Livepatch symbols must be marked with SHN_LIVEPATCH so that the module +loader can identify and ignore them. Livepatch modules keep these symbols +in their symbol tables, and the symbol table is made accessible through +module->symtab. + +------------------------------------- +3.2 A livepatch module's symbol table +------------------------------------- +Normally, a stripped down copy of a module's symbol table (containing only +"core" symbols) is made available through module->symtab (See layout_symtab() +in kernel/module.c). For livepatch modules, the symbol table copied into memory +on module load must be exactly the same as the symbol table produced when the +patch module was compiled. This is because the relocations in each livepatch +relocation section refer to their respective symbols with their symbol indices, +and the original symbol indices (and thus the symtab ordering) must be +preserved in order for apply_relocate_add() to find the right symbol. + +For example, take this particular rela from a livepatch module: +Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries: + Offset Info Type Symbol's Value Symbol's Name + Addend +000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4 + +This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded +in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the +symbol index 94. +And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol: +[ snip ] +94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0 +[ snip ] + +--------------------------- +3.3 Livepatch symbol format +--------------------------- + +3.3.1 Required flags +-------------------- +Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so +that the module loader can identify them and not attempt to resolve them. +See include/uapi/linux/elf.h for the actual definitions. + +3.3.2 Required name format +-------------------------- +Livepatch symbol names must conform to the following format: + +.klp.sym.objname.symbol_name,sympos +^ ^^ ^ ^ ^ ^ +|_______||_____| |_________| | + [A] [B] [C] [D] + +[A] The symbol name is prefixed with the string ".klp.sym." +[B] The name of the object (i.e. "vmlinux" or name of module) to + which the symbol belongs follows immediately after the prefix. +[C] The actual name of the symbol. +[D] The position of the symbol in the object (as according to kallsyms) + This is used to differentiate duplicate symbols within the same + object. The symbol position is expressed numerically (0, 1, 2...). + The symbol position of a unique symbol is 0. + +3.3.3 Example livepatch symbol names: +------------------------------------- +.klp.sym.vmlinux.snprintf,0 +.klp.sym.vmlinux.printk,0 +.klp.sym.btrfs.btrfs_ktype,0 + +3.3.4 Example `readelf --symbols` output for a patch module: +------------------------------------------------------------ +Symbol table '.symtab' contains 127 entries: + Num: Value Size Type Bind Vis Ndx Name + [ snip ] + 73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0 + 74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0 + 75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0 + 76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0 + [ snip ] ^ + | + [*] +[*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20). + "OS" means OS-specific. + +--------------------------------- +4. Architecture-specific sections +--------------------------------- +Architectures may override arch_klp_init_object_loaded() to perform +additional arch-specific tasks when a target module loads, such as applying +arch-specific sections. On x86 for example, we must apply per-object +.altinstructions and .parainstructions sections when a target module loads. +These sections must be prefixed with ".klp.arch.$objname." so that they can +be easily identified when iterating through a patch module's Elf sections +(See arch/x86/kernel/livepatch.c for a complete example). + +-------------------------------------- +5. Symbol table and Elf section access +-------------------------------------- +A livepatch module's symbol table is accessible through module->symtab. + +Since apply_relocate_add() requires access to a module's section headers, +symbol table, and relocation section indices, Elf information is preserved for +livepatch modules and is made accessible by the module loader through +module->klp_info, which is a klp_modinfo struct. When a livepatch module loads, +this struct is filled in by the module loader. Its fields are documented below: + +struct klp_modinfo { + Elf_Ehdr hdr; /* Elf header */ + Elf_Shdr *sechdrs; /* Section header table */ + char *secstrings; /* String table for the section headers */ + unsigned int symndx; /* The symbol table section index */ +}; diff --git a/Documentation/livepatch/shadow-vars.txt b/Documentation/livepatch/shadow-vars.txt new file mode 100644 index 000000000..ecc09a7be --- /dev/null +++ b/Documentation/livepatch/shadow-vars.txt @@ -0,0 +1,209 @@ +================ +Shadow Variables +================ + +Shadow variables are a simple way for livepatch modules to associate +additional "shadow" data with existing data structures. Shadow data is +allocated separately from parent data structures, which are left +unmodified. The shadow variable API described in this document is used +to allocate/add and remove/free shadow variables to/from their parents. + +The implementation introduces a global, in-kernel hashtable that +associates pointers to parent objects and a numeric identifier of the +shadow data. The numeric identifier is a simple enumeration that may be +used to describe shadow variable version, class or type, etc. More +specifically, the parent pointer serves as the hashtable key while the +numeric id subsequently filters hashtable queries. Multiple shadow +variables may attach to the same parent object, but their numeric +identifier distinguishes between them. + + +1. Brief API summary +==================== + +(See the full API usage docbook notes in livepatch/shadow.c.) + +A hashtable references all shadow variables. These references are +stored and retrieved through a <obj, id> pair. + +* The klp_shadow variable data structure encapsulates both tracking +meta-data and shadow-data: + - meta-data + - obj - pointer to parent object + - id - data identifier + - data[] - storage for shadow data + +It is important to note that the klp_shadow_alloc() and +klp_shadow_get_or_alloc() are zeroing the variable by default. +They also allow to call a custom constructor function when a non-zero +value is needed. Callers should provide whatever mutual exclusion +is required. + +Note that the constructor is called under klp_shadow_lock spinlock. It allows +to do actions that can be done only once when a new variable is allocated. + +* klp_shadow_get() - retrieve a shadow variable data pointer + - search hashtable for <obj, id> pair + +* klp_shadow_alloc() - allocate and add a new shadow variable + - search hashtable for <obj, id> pair + - if exists + - WARN and return NULL + - if <obj, id> doesn't already exist + - allocate a new shadow variable + - initialize the variable using a custom constructor and data when provided + - add <obj, id> to the global hashtable + +* klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable + - search hashtable for <obj, id> pair + - if exists + - return existing shadow variable + - if <obj, id> doesn't already exist + - allocate a new shadow variable + - initialize the variable using a custom constructor and data when provided + - add <obj, id> pair to the global hashtable + +* klp_shadow_free() - detach and free a <obj, id> shadow variable + - find and remove a <obj, id> reference from global hashtable + - if found + - call destructor function if defined + - free shadow variable + +* klp_shadow_free_all() - detach and free all <*, id> shadow variables + - find and remove any <*, id> references from global hashtable + - if found + - call destructor function if defined + - free shadow variable + + +2. Use cases +============ + +(See the example shadow variable livepatch modules in samples/livepatch/ +for full working demonstrations.) + +For the following use-case examples, consider commit 1d147bfa6429 +("mac80211: fix AP powersave TX vs. wakeup race"), which added a +spinlock to net/mac80211/sta_info.h :: struct sta_info. Each use-case +example can be considered a stand-alone livepatch implementation of this +fix. + + +Matching parent's lifecycle +--------------------------- + +If parent data structures are frequently created and destroyed, it may +be easiest to align their shadow variables lifetimes to the same +allocation and release functions. In this case, the parent data +structure is typically allocated, initialized, then registered in some +manner. Shadow variable allocation and setup can then be considered +part of the parent's initialization and should be completed before the +parent "goes live" (ie, any shadow variable get-API requests are made +for this <obj, id> pair.) + +For commit 1d147bfa6429, when a parent sta_info structure is allocated, +allocate a shadow copy of the ps_lock pointer, then initialize it: + +#define PS_LOCK 1 +struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata, + const u8 *addr, gfp_t gfp) +{ + struct sta_info *sta; + spinlock_t *ps_lock; + + /* Parent structure is created */ + sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp); + + /* Attach a corresponding shadow variable, then initialize it */ + ps_lock = klp_shadow_alloc(sta, PS_LOCK, sizeof(*ps_lock), gfp, + NULL, NULL); + if (!ps_lock) + goto shadow_fail; + spin_lock_init(ps_lock); + ... + +When requiring a ps_lock, query the shadow variable API to retrieve one +for a specific struct sta_info: + +void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta) +{ + spinlock_t *ps_lock; + + /* sync with ieee80211_tx_h_unicast_ps_buf */ + ps_lock = klp_shadow_get(sta, PS_LOCK); + if (ps_lock) + spin_lock(ps_lock); + ... + +When the parent sta_info structure is freed, first free the shadow +variable: + +void sta_info_free(struct ieee80211_local *local, struct sta_info *sta) +{ + klp_shadow_free(sta, PS_LOCK, NULL); + kfree(sta); + ... + + +In-flight parent objects +------------------------ + +Sometimes it may not be convenient or possible to allocate shadow +variables alongside their parent objects. Or a livepatch fix may +require shadow varibles to only a subset of parent object instances. In +these cases, the klp_shadow_get_or_alloc() call can be used to attach +shadow variables to parents already in-flight. + +For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is +inside ieee80211_sta_ps_deliver_wakeup(): + +int ps_lock_shadow_ctor(void *obj, void *shadow_data, void *ctor_data) +{ + spinlock_t *lock = shadow_data; + + spin_lock_init(lock); + return 0; +} + +#define PS_LOCK 1 +void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta) +{ + spinlock_t *ps_lock; + + /* sync with ieee80211_tx_h_unicast_ps_buf */ + ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK, + sizeof(*ps_lock), GFP_ATOMIC, + ps_lock_shadow_ctor, NULL); + + if (ps_lock) + spin_lock(ps_lock); + ... + +This usage will create a shadow variable, only if needed, otherwise it +will use one that was already created for this <obj, id> pair. + +Like the previous use-case, the shadow spinlock needs to be cleaned up. +A shadow variable can be freed just before its parent object is freed, +or even when the shadow variable itself is no longer required. + + +Other use-cases +--------------- + +Shadow variables can also be used as a flag indicating that a data +structure was allocated by new, livepatched code. In this case, it +doesn't matter what data value the shadow variable holds, its existence +suggests how to handle the parent object. + + +3. References +============= + +* https://github.com/dynup/kpatch +The livepatch implementation is based on the kpatch version of shadow +variables. + +* http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf +Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity +Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented +a datatype update technique called "shadow data structures". |