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diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst new file mode 100644 index 000000000..910668e66 --- /dev/null +++ b/Documentation/security/self-protection.rst @@ -0,0 +1,316 @@ +====================== +Kernel Self-Protection +====================== + +Kernel self-protection is the design and implementation of systems and +structures within the Linux kernel to protect against security flaws in +the kernel itself. This covers a wide range of issues, including removing +entire classes of bugs, blocking security flaw exploitation methods, +and actively detecting attack attempts. Not all topics are explored in +this document, but it should serve as a reasonable starting point and +answer any frequently asked questions. (Patches welcome, of course!) + +In the worst-case scenario, we assume an unprivileged local attacker +has arbitrary read and write access to the kernel's memory. In many +cases, bugs being exploited will not provide this level of access, +but with systems in place that defend against the worst case we'll +cover the more limited cases as well. A higher bar, and one that should +still be kept in mind, is protecting the kernel against a _privileged_ +local attacker, since the root user has access to a vastly increased +attack surface. (Especially when they have the ability to load arbitrary +kernel modules.) + +The goals for successful self-protection systems would be that they +are effective, on by default, require no opt-in by developers, have no +performance impact, do not impede kernel debugging, and have tests. It +is uncommon that all these goals can be met, but it is worth explicitly +mentioning them, since these aspects need to be explored, dealt with, +and/or accepted. + + +Attack Surface Reduction +======================== + +The most fundamental defense against security exploits is to reduce the +areas of the kernel that can be used to redirect execution. This ranges +from limiting the exposed APIs available to userspace, making in-kernel +APIs hard to use incorrectly, minimizing the areas of writable kernel +memory, etc. + +Strict kernel memory permissions +-------------------------------- + +When all of kernel memory is writable, it becomes trivial for attacks +to redirect execution flow. To reduce the availability of these targets +the kernel needs to protect its memory with a tight set of permissions. + +Executable code and read-only data must not be writable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Any areas of the kernel with executable memory must not be writable. +While this obviously includes the kernel text itself, we must consider +all additional places too: kernel modules, JIT memory, etc. (There are +temporary exceptions to this rule to support things like instruction +alternatives, breakpoints, kprobes, etc. If these must exist in a +kernel, they are implemented in a way where the memory is temporarily +made writable during the update, and then returned to the original +permissions.) + +In support of this are ``CONFIG_STRICT_KERNEL_RWX`` and +``CONFIG_STRICT_MODULE_RWX``, which seek to make sure that code is not +writable, data is not executable, and read-only data is neither writable +nor executable. + +Most architectures have these options on by default and not user selectable. +For some architectures like arm that wish to have these be selectable, +the architecture Kconfig can select ARCH_OPTIONAL_KERNEL_RWX to enable +a Kconfig prompt. ``CONFIG_ARCH_OPTIONAL_KERNEL_RWX_DEFAULT`` determines +the default setting when ARCH_OPTIONAL_KERNEL_RWX is enabled. + +Function pointers and sensitive variables must not be writable +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Vast areas of kernel memory contain function pointers that are looked +up by the kernel and used to continue execution (e.g. descriptor/vector +tables, file/network/etc operation structures, etc). The number of these +variables must be reduced to an absolute minimum. + +Many such variables can be made read-only by setting them "const" +so that they live in the .rodata section instead of the .data section +of the kernel, gaining the protection of the kernel's strict memory +permissions as described above. + +For variables that are initialized once at ``__init`` time, these can +be marked with the ``__ro_after_init`` attribute. + +What remains are variables that are updated rarely (e.g. GDT). These +will need another infrastructure (similar to the temporary exceptions +made to kernel code mentioned above) that allow them to spend the rest +of their lifetime read-only. (For example, when being updated, only the +CPU thread performing the update would be given uninterruptible write +access to the memory.) + +Segregation of kernel memory from userspace memory +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The kernel must never execute userspace memory. The kernel must also never +access userspace memory without explicit expectation to do so. These +rules can be enforced either by support of hardware-based restrictions +(x86's SMEP/SMAP, ARM's PXN/PAN) or via emulation (ARM's Memory Domains). +By blocking userspace memory in this way, execution and data parsing +cannot be passed to trivially-controlled userspace memory, forcing +attacks to operate entirely in kernel memory. + +Reduced access to syscalls +-------------------------- + +One trivial way to eliminate many syscalls for 64-bit systems is building +without ``CONFIG_COMPAT``. However, this is rarely a feasible scenario. + +The "seccomp" system provides an opt-in feature made available to +userspace, which provides a way to reduce the number of kernel entry +points available to a running process. This limits the breadth of kernel +code that can be reached, possibly reducing the availability of a given +bug to an attack. + +An area of improvement would be creating viable ways to keep access to +things like compat, user namespaces, BPF creation, and perf limited only +to trusted processes. This would keep the scope of kernel entry points +restricted to the more regular set of normally available to unprivileged +userspace. + +Restricting access to kernel modules +------------------------------------ + +The kernel should never allow an unprivileged user the ability to +load specific kernel modules, since that would provide a facility to +unexpectedly extend the available attack surface. (The on-demand loading +of modules via their predefined subsystems, e.g. MODULE_ALIAS_*, is +considered "expected" here, though additional consideration should be +given even to these.) For example, loading a filesystem module via an +unprivileged socket API is nonsense: only the root or physically local +user should trigger filesystem module loading. (And even this can be up +for debate in some scenarios.) + +To protect against even privileged users, systems may need to either +disable module loading entirely (e.g. monolithic kernel builds or +modules_disabled sysctl), or provide signed modules (e.g. +``CONFIG_MODULE_SIG_FORCE``, or dm-crypt with LoadPin), to keep from having +root load arbitrary kernel code via the module loader interface. + + +Memory integrity +================ + +There are many memory structures in the kernel that are regularly abused +to gain execution control during an attack, By far the most commonly +understood is that of the stack buffer overflow in which the return +address stored on the stack is overwritten. Many other examples of this +kind of attack exist, and protections exist to defend against them. + +Stack buffer overflow +--------------------- + +The classic stack buffer overflow involves writing past the expected end +of a variable stored on the stack, ultimately writing a controlled value +to the stack frame's stored return address. The most widely used defense +is the presence of a stack canary between the stack variables and the +return address (``CONFIG_STACKPROTECTOR``), which is verified just before +the function returns. Other defenses include things like shadow stacks. + +Stack depth overflow +-------------------- + +A less well understood attack is using a bug that triggers the +kernel to consume stack memory with deep function calls or large stack +allocations. With this attack it is possible to write beyond the end of +the kernel's preallocated stack space and into sensitive structures. Two +important changes need to be made for better protections: moving the +sensitive thread_info structure elsewhere, and adding a faulting memory +hole at the bottom of the stack to catch these overflows. + +Heap memory integrity +--------------------- + +The structures used to track heap free lists can be sanity-checked during +allocation and freeing to make sure they aren't being used to manipulate +other memory areas. + +Counter integrity +----------------- + +Many places in the kernel use atomic counters to track object references +or perform similar lifetime management. When these counters can be made +to wrap (over or under) this traditionally exposes a use-after-free +flaw. By trapping atomic wrapping, this class of bug vanishes. + +Size calculation overflow detection +----------------------------------- + +Similar to counter overflow, integer overflows (usually size calculations) +need to be detected at runtime to kill this class of bug, which +traditionally leads to being able to write past the end of kernel buffers. + + +Probabilistic defenses +====================== + +While many protections can be considered deterministic (e.g. read-only +memory cannot be written to), some protections provide only statistical +defense, in that an attack must gather enough information about a +running system to overcome the defense. While not perfect, these do +provide meaningful defenses. + +Canaries, blinding, and other secrets +------------------------------------- + +It should be noted that things like the stack canary discussed earlier +are technically statistical defenses, since they rely on a secret value, +and such values may become discoverable through an information exposure +flaw. + +Blinding literal values for things like JITs, where the executable +contents may be partially under the control of userspace, need a similar +secret value. + +It is critical that the secret values used must be separate (e.g. +different canary per stack) and high entropy (e.g. is the RNG actually +working?) in order to maximize their success. + +Kernel Address Space Layout Randomization (KASLR) +------------------------------------------------- + +Since the location of kernel memory is almost always instrumental in +mounting a successful attack, making the location non-deterministic +raises the difficulty of an exploit. (Note that this in turn makes +the value of information exposures higher, since they may be used to +discover desired memory locations.) + +Text and module base +~~~~~~~~~~~~~~~~~~~~ + +By relocating the physical and virtual base address of the kernel at +boot-time (``CONFIG_RANDOMIZE_BASE``), attacks needing kernel code will be +frustrated. Additionally, offsetting the module loading base address +means that even systems that load the same set of modules in the same +order every boot will not share a common base address with the rest of +the kernel text. + +Stack base +~~~~~~~~~~ + +If the base address of the kernel stack is not the same between processes, +or even not the same between syscalls, targets on or beyond the stack +become more difficult to locate. + +Dynamic memory base +~~~~~~~~~~~~~~~~~~~ + +Much of the kernel's dynamic memory (e.g. kmalloc, vmalloc, etc) ends up +being relatively deterministic in layout due to the order of early-boot +initializations. If the base address of these areas is not the same +between boots, targeting them is frustrated, requiring an information +exposure specific to the region. + +Structure layout +~~~~~~~~~~~~~~~~ + +By performing a per-build randomization of the layout of sensitive +structures, attacks must either be tuned to known kernel builds or expose +enough kernel memory to determine structure layouts before manipulating +them. + + +Preventing Information Exposures +================================ + +Since the locations of sensitive structures are the primary target for +attacks, it is important to defend against exposure of both kernel memory +addresses and kernel memory contents (since they may contain kernel +addresses or other sensitive things like canary values). + +Kernel addresses +---------------- + +Printing kernel addresses to userspace leaks sensitive information about +the kernel memory layout. Care should be exercised when using any printk +specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb] +in certain circumstances [*]). Any file written to using one of these +specifiers should be readable only by privileged processes. + +Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1 +addresses printed with the specifier %p are hashed before printing. + +[*] If KALLSYMS is enabled and symbol lookup fails, the raw address is +printed. If KALLSYMS is not enabled the raw address is printed. + +Unique identifiers +------------------ + +Kernel memory addresses must never be used as identifiers exposed to +userspace. Instead, use an atomic counter, an idr, or similar unique +identifier. + +Memory initialization +--------------------- + +Memory copied to userspace must always be fully initialized. If not +explicitly memset(), this will require changes to the compiler to make +sure structure holes are cleared. + +Memory poisoning +---------------- + +When releasing memory, it is best to poison the contents, to avoid reuse +attacks that rely on the old contents of memory. E.g., clear stack on a +syscall return (``CONFIG_GCC_PLUGIN_STACKLEAK``), wipe heap memory on a +free. This frustrates many uninitialized variable attacks, stack content +exposures, heap content exposures, and use-after-free attacks. + +Destination tracking +-------------------- + +To help kill classes of bugs that result in kernel addresses being +written to userspace, the destination of writes needs to be tracked. If +the buffer is destined for userspace (e.g. seq_file backed ``/proc`` files), +it should automatically censor sensitive values. |