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doc: ReSTify self-protection.txt
This updates the credentials API documentation to ReST markup and moves it under the security subsection of kernel API documentation. Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Kees Cook
Jonathan Corbet
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@ -7,4 +7,5 @@ Security Documentation
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credentials
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IMA-templates
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self-protection
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tpm/index
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@ -1,4 +1,6 @@
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# Kernel Self-Protection
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======================
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Kernel Self-Protection
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======================
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Kernel self-protection is the design and implementation of systems and
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structures within the Linux kernel to protect against security flaws in
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@ -26,7 +28,8 @@ mentioning them, since these aspects need to be explored, dealt with,
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and/or accepted.
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## Attack Surface Reduction
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Attack Surface Reduction
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========================
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The most fundamental defense against security exploits is to reduce the
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areas of the kernel that can be used to redirect execution. This ranges
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@ -34,13 +37,15 @@ from limiting the exposed APIs available to userspace, making in-kernel
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APIs hard to use incorrectly, minimizing the areas of writable kernel
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memory, etc.
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### Strict kernel memory permissions
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Strict kernel memory permissions
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--------------------------------
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When all of kernel memory is writable, it becomes trivial for attacks
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to redirect execution flow. To reduce the availability of these targets
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the kernel needs to protect its memory with a tight set of permissions.
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#### Executable code and read-only data must not be writable
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Executable code and read-only data must not be writable
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Any areas of the kernel with executable memory must not be writable.
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While this obviously includes the kernel text itself, we must consider
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@ -51,18 +56,19 @@ kernel, they are implemented in a way where the memory is temporarily
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made writable during the update, and then returned to the original
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permissions.)
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In support of this are CONFIG_STRICT_KERNEL_RWX and
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CONFIG_STRICT_MODULE_RWX, which seek to make sure that code is not
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In support of this are ``CONFIG_STRICT_KERNEL_RWX`` and
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``CONFIG_STRICT_MODULE_RWX``, which seek to make sure that code is not
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writable, data is not executable, and read-only data is neither writable
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nor executable.
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Most architectures have these options on by default and not user selectable.
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For some architectures like arm that wish to have these be selectable,
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the architecture Kconfig can select ARCH_OPTIONAL_KERNEL_RWX to enable
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a Kconfig prompt. CONFIG_ARCH_OPTIONAL_KERNEL_RWX_DEFAULT determines
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a Kconfig prompt. ``CONFIG_ARCH_OPTIONAL_KERNEL_RWX_DEFAULT`` determines
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the default setting when ARCH_OPTIONAL_KERNEL_RWX is enabled.
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#### Function pointers and sensitive variables must not be writable
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Function pointers and sensitive variables must not be writable
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Vast areas of kernel memory contain function pointers that are looked
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up by the kernel and used to continue execution (e.g. descriptor/vector
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@ -74,8 +80,8 @@ so that they live in the .rodata section instead of the .data section
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of the kernel, gaining the protection of the kernel's strict memory
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permissions as described above.
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For variables that are initialized once at __init time, these can
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be marked with the (new and under development) __ro_after_init
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For variables that are initialized once at ``__init`` time, these can
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be marked with the (new and under development) ``__ro_after_init``
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attribute.
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What remains are variables that are updated rarely (e.g. GDT). These
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@ -85,7 +91,8 @@ of their lifetime read-only. (For example, when being updated, only the
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CPU thread performing the update would be given uninterruptible write
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access to the memory.)
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#### Segregation of kernel memory from userspace memory
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Segregation of kernel memory from userspace memory
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The kernel must never execute userspace memory. The kernel must also never
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access userspace memory without explicit expectation to do so. These
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@ -95,10 +102,11 @@ By blocking userspace memory in this way, execution and data parsing
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cannot be passed to trivially-controlled userspace memory, forcing
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attacks to operate entirely in kernel memory.
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### Reduced access to syscalls
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Reduced access to syscalls
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--------------------------
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One trivial way to eliminate many syscalls for 64-bit systems is building
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without CONFIG_COMPAT. However, this is rarely a feasible scenario.
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without ``CONFIG_COMPAT``. However, this is rarely a feasible scenario.
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The "seccomp" system provides an opt-in feature made available to
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userspace, which provides a way to reduce the number of kernel entry
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@ -112,7 +120,8 @@ to trusted processes. This would keep the scope of kernel entry points
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restricted to the more regular set of normally available to unprivileged
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userspace.
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### Restricting access to kernel modules
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Restricting access to kernel modules
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------------------------------------
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The kernel should never allow an unprivileged user the ability to
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load specific kernel modules, since that would provide a facility to
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@ -127,11 +136,12 @@ for debate in some scenarios.)
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To protect against even privileged users, systems may need to either
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disable module loading entirely (e.g. monolithic kernel builds or
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modules_disabled sysctl), or provide signed modules (e.g.
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CONFIG_MODULE_SIG_FORCE, or dm-crypt with LoadPin), to keep from having
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``CONFIG_MODULE_SIG_FORCE``, or dm-crypt with LoadPin), to keep from having
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root load arbitrary kernel code via the module loader interface.
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## Memory integrity
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Memory integrity
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================
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There are many memory structures in the kernel that are regularly abused
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to gain execution control during an attack, By far the most commonly
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@ -139,16 +149,18 @@ understood is that of the stack buffer overflow in which the return
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address stored on the stack is overwritten. Many other examples of this
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kind of attack exist, and protections exist to defend against them.
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### Stack buffer overflow
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Stack buffer overflow
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---------------------
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The classic stack buffer overflow involves writing past the expected end
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of a variable stored on the stack, ultimately writing a controlled value
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to the stack frame's stored return address. The most widely used defense
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is the presence of a stack canary between the stack variables and the
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return address (CONFIG_CC_STACKPROTECTOR), which is verified just before
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return address (``CONFIG_CC_STACKPROTECTOR``), which is verified just before
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the function returns. Other defenses include things like shadow stacks.
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### Stack depth overflow
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Stack depth overflow
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--------------------
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A less well understood attack is using a bug that triggers the
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kernel to consume stack memory with deep function calls or large stack
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@ -158,27 +170,31 @@ important changes need to be made for better protections: moving the
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sensitive thread_info structure elsewhere, and adding a faulting memory
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hole at the bottom of the stack to catch these overflows.
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### Heap memory integrity
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Heap memory integrity
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---------------------
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The structures used to track heap free lists can be sanity-checked during
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allocation and freeing to make sure they aren't being used to manipulate
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other memory areas.
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### Counter integrity
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Counter integrity
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-----------------
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Many places in the kernel use atomic counters to track object references
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or perform similar lifetime management. When these counters can be made
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to wrap (over or under) this traditionally exposes a use-after-free
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flaw. By trapping atomic wrapping, this class of bug vanishes.
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### Size calculation overflow detection
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Size calculation overflow detection
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-----------------------------------
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Similar to counter overflow, integer overflows (usually size calculations)
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need to be detected at runtime to kill this class of bug, which
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traditionally leads to being able to write past the end of kernel buffers.
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## Statistical defenses
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Probabilistic defenses
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======================
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While many protections can be considered deterministic (e.g. read-only
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memory cannot be written to), some protections provide only statistical
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@ -186,7 +202,8 @@ defense, in that an attack must gather enough information about a
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running system to overcome the defense. While not perfect, these do
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provide meaningful defenses.
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### Canaries, blinding, and other secrets
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Canaries, blinding, and other secrets
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-------------------------------------
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It should be noted that things like the stack canary discussed earlier
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are technically statistical defenses, since they rely on a secret value,
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@ -201,7 +218,8 @@ It is critical that the secret values used must be separate (e.g.
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different canary per stack) and high entropy (e.g. is the RNG actually
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working?) in order to maximize their success.
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### Kernel Address Space Layout Randomization (KASLR)
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Kernel Address Space Layout Randomization (KASLR)
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-------------------------------------------------
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Since the location of kernel memory is almost always instrumental in
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mounting a successful attack, making the location non-deterministic
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@ -209,22 +227,25 @@ raises the difficulty of an exploit. (Note that this in turn makes
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the value of information exposures higher, since they may be used to
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discover desired memory locations.)
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#### Text and module base
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Text and module base
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~~~~~~~~~~~~~~~~~~~~
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By relocating the physical and virtual base address of the kernel at
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boot-time (CONFIG_RANDOMIZE_BASE), attacks needing kernel code will be
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boot-time (``CONFIG_RANDOMIZE_BASE``), attacks needing kernel code will be
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frustrated. Additionally, offsetting the module loading base address
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means that even systems that load the same set of modules in the same
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order every boot will not share a common base address with the rest of
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the kernel text.
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#### Stack base
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Stack base
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~~~~~~~~~~
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If the base address of the kernel stack is not the same between processes,
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or even not the same between syscalls, targets on or beyond the stack
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become more difficult to locate.
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#### Dynamic memory base
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Dynamic memory base
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~~~~~~~~~~~~~~~~~~~
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Much of the kernel's dynamic memory (e.g. kmalloc, vmalloc, etc) ends up
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being relatively deterministic in layout due to the order of early-boot
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@ -232,7 +253,8 @@ initializations. If the base address of these areas is not the same
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between boots, targeting them is frustrated, requiring an information
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exposure specific to the region.
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#### Structure layout
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Structure layout
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~~~~~~~~~~~~~~~~
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By performing a per-build randomization of the layout of sensitive
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structures, attacks must either be tuned to known kernel builds or expose
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@ -240,26 +262,30 @@ enough kernel memory to determine structure layouts before manipulating
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them.
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## Preventing Information Exposures
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Preventing Information Exposures
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================================
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Since the locations of sensitive structures are the primary target for
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attacks, it is important to defend against exposure of both kernel memory
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addresses and kernel memory contents (since they may contain kernel
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addresses or other sensitive things like canary values).
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### Unique identifiers
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Unique identifiers
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------------------
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Kernel memory addresses must never be used as identifiers exposed to
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userspace. Instead, use an atomic counter, an idr, or similar unique
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identifier.
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### Memory initialization
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Memory initialization
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---------------------
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Memory copied to userspace must always be fully initialized. If not
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explicitly memset(), this will require changes to the compiler to make
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sure structure holes are cleared.
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### Memory poisoning
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Memory poisoning
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----------------
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When releasing memory, it is best to poison the contents (clear stack on
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syscall return, wipe heap memory on a free), to avoid reuse attacks that
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@ -267,9 +293,10 @@ rely on the old contents of memory. This frustrates many uninitialized
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variable attacks, stack content exposures, heap content exposures, and
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use-after-free attacks.
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### Destination tracking
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Destination tracking
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--------------------
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To help kill classes of bugs that result in kernel addresses being
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written to userspace, the destination of writes needs to be tracked. If
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the buffer is destined for userspace (e.g. seq_file backed /proc files),
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the buffer is destined for userspace (e.g. seq_file backed ``/proc`` files),
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it should automatically censor sensitive values.
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