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| -rw-r--r-- | Documentation/security/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/security/self-protection.rst (renamed from Documentation/security/self-protection.txt) | 99 |
2 files changed, 64 insertions, 36 deletions
diff --git a/Documentation/security/index.rst b/Documentation/security/index.rst index 415be8e0b013..4212d7ac58b6 100644 --- a/Documentation/security/index.rst +++ b/Documentation/security/index.rst @@ -7,4 +7,5 @@ Security Documentation credentials IMA-templates + self-protection tpm/index diff --git a/Documentation/security/self-protection.txt b/Documentation/security/self-protection.rst index 141acfebe6ef..60c8bd8b77bf 100644 --- a/Documentation/security/self-protection.txt +++ b/Documentation/security/self-protection.rst @@ -1,4 +1,6 @@ -# Kernel Self-Protection +====================== +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 @@ -26,7 +28,8 @@ mentioning them, since these aspects need to be explored, dealt with, and/or accepted. -## Attack Surface Reduction +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 @@ -34,13 +37,15 @@ 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 +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 +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 @@ -51,18 +56,19 @@ 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 +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 +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 +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 @@ -74,8 +80,8 @@ 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 (new and under development) __ro_after_init +For variables that are initialized once at ``__init`` time, these can +be marked with the (new and under development) ``__ro_after_init`` attribute. What remains are variables that are updated rarely (e.g. GDT). These @@ -85,7 +91,8 @@ 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 +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 @@ -95,10 +102,11 @@ 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 +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. +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 @@ -112,7 +120,8 @@ 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 +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 @@ -127,11 +136,12 @@ 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 +``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 +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 @@ -139,16 +149,18 @@ 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 +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_CC_STACKPROTECTOR), which is verified just before +return address (``CONFIG_CC_STACKPROTECTOR``), which is verified just before the function returns. Other defenses include things like shadow stacks. -### Stack depth overflow +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 @@ -158,27 +170,31 @@ 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 +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 +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 +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. -## Statistical defenses +Probabilistic defenses +====================== While many protections can be considered deterministic (e.g. read-only memory cannot be written to), some protections provide only statistical @@ -186,7 +202,8 @@ 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 +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, @@ -201,7 +218,8 @@ 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) +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 @@ -209,22 +227,25 @@ 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 +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 +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 +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 +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 @@ -232,7 +253,8 @@ 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 +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 @@ -240,26 +262,30 @@ enough kernel memory to determine structure layouts before manipulating them. -## Preventing Information Exposures +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). -### Unique identifiers +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 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 +Memory poisoning +---------------- When releasing memory, it is best to poison the contents (clear stack on syscall return, wipe heap memory on a free), to avoid reuse attacks that @@ -267,9 +293,10 @@ rely on the old contents of memory. This frustrates many uninitialized variable attacks, stack content exposures, heap content exposures, and use-after-free attacks. -### Destination tracking +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), +the buffer is destined for userspace (e.g. seq_file backed ``/proc`` files), it should automatically censor sensitive values. |