Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+=========
+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.