doc: self-protection: provide initial details
This document attempts to codify the intent around kernel self-protection along with discussion of both existing and desired technologies, with attention given to the rationale behind them, and the expectations of their usage. Signed-off-by: Kees Cook <keescook@chromium.org> Reviewed-by: Randy Dunlap <rdunlap@infradead.org> [jc: applied fixes suggested by Randy] Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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# Kernel Self-Protection
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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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the kernel itself. This covers a wide range of issues, including removing
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entire classes of bugs, blocking security flaw exploitation methods,
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and actively detecting attack attempts. Not all topics are explored in
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this document, but it should serve as a reasonable starting point and
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answer any frequently asked questions. (Patches welcome, of course!)
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In the worst-case scenario, we assume an unprivileged local attacker
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has arbitrary read and write access to the kernel's memory. In many
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cases, bugs being exploited will not provide this level of access,
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but with systems in place that defend against the worst case we'll
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cover the more limited cases as well. A higher bar, and one that should
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still be kept in mind, is protecting the kernel against a _privileged_
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local attacker, since the root user has access to a vastly increased
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attack surface. (Especially when they have the ability to load arbitrary
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kernel modules.)
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The goals for successful self-protection systems would be that they
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are effective, on by default, require no opt-in by developers, have no
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performance impact, do not impede kernel debugging, and have tests. It
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is uncommon that all these goals can be met, but it is worth explicitly
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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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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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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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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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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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all additional places too: kernel modules, JIT memory, etc. (There are
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temporary exceptions to this rule to support things like instruction
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alternatives, breakpoints, kprobes, etc. If these must exist in a
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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 (the poorly named) CONFIG_DEBUG_RODATA and
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CONFIG_DEBUG_SET_MODULE_RONX, 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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#### Function pointers and sensitive variables must not be writable
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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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tables, file/network/etc operation structures, etc). The number of these
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variables must be reduced to an absolute minimum.
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Many such variables can be made read-only by setting them "const"
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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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attribute.
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What remains are variables that are updated rarely (e.g. GDT). These
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will need another infrastructure (similar to the temporary exceptions
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made to kernel code mentioned above) that allow them to spend the rest
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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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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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rules can be enforced either by support of hardware-based restrictions
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(x86's SMEP/SMAP, ARM's PXN/PAN) or via emulation (ARM's Memory Domains).
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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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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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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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points available to a running process. This limits the breadth of kernel
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code that can be reached, possibly reducing the availability of a given
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bug to an attack.
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An area of improvement would be creating viable ways to keep access to
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things like compat, user namespaces, BPF creation, and perf limited only
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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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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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unexpectedly extend the available attack surface. (The on-demand loading
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of modules via their predefined subsystems, e.g. MODULE_ALIAS_*, is
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considered "expected" here, though additional consideration should be
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given even to these.) For example, loading a filesystem module via an
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unprivileged socket API is nonsense: only the root or physically local
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user should trigger filesystem module loading. (And even this can be up
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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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root load arbitrary kernel code via the module loader interface.
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## Memory integrity
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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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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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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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the function returns. Other defenses include things like shadow stacks.
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### Stack depth overflow
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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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allocations. With this attack it is possible to write beyond the end of
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the kernel's preallocated stack space and into sensitive structures. Two
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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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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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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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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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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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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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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 (leakable)
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secret value.
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Blinding literal values for things like JITs, where the executable
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contents may be partially under the control of userspace, need a similar
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secret value.
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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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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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raises the difficulty of an exploit. (Note that this in turn makes
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the value of leaks higher, since they may be used to discover desired
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memory locations.)
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#### Text and module base
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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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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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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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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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initializations. If the base address of these areas is not the same
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between boots, targeting them is frustrated, requiring a leak specific
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to the region.
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## Preventing Leaks
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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 leaks 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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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 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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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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rely on the old contents of memory. This frustrates many uninitialized
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variable attacks, stack info leaks, heap info leaks, and use-after-free
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attacks.
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### Destination tracking
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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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it should automatically censor sensitive values.
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