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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: NKees Cook <keescook@chromium.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>
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Documentation/security/index.rst
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...@@ -7,4 +7,5 @@ Security Documentation ...@@ -7,4 +7,5 @@ Security Documentation
credentials credentials
IMA-templates IMA-templates
self-protection
tpm/index tpm/index
Documentation/security/self-protection.txt → Documentation/security/self-protection.rst
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# Kernel Self-Protection ======================
Kernel Self-Protection
======================
Kernel self-protection is the design and implementation of systems and Kernel self-protection is the design and implementation of systems and
structures within the Linux kernel to protect against security flaws in 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, ...@@ -26,7 +28,8 @@ mentioning them, since these aspects need to be explored, dealt with,
and/or accepted. and/or accepted.
## Attack Surface Reduction Attack Surface Reduction
========================
The most fundamental defense against security exploits is to reduce the The most fundamental defense against security exploits is to reduce the
areas of the kernel that can be used to redirect execution. This ranges 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 ...@@ -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 APIs hard to use incorrectly, minimizing the areas of writable kernel
memory, etc. memory, etc.
### Strict kernel memory permissions Strict kernel memory permissions
--------------------------------
When all of kernel memory is writable, it becomes trivial for attacks When all of kernel memory is writable, it becomes trivial for attacks
to redirect execution flow. To reduce the availability of these targets to redirect execution flow. To reduce the availability of these targets
the kernel needs to protect its memory with a tight set of permissions. 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. Any areas of the kernel with executable memory must not be writable.
While this obviously includes the kernel text itself, we must consider 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 ...@@ -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 made writable during the update, and then returned to the original
permissions.) permissions.)
In support of this are CONFIG_STRICT_KERNEL_RWX and In support of this are ``CONFIG_STRICT_KERNEL_RWX`` and
CONFIG_STRICT_MODULE_RWX, which seek to make sure that code is not ``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 writable, data is not executable, and read-only data is neither writable
nor executable. nor executable.
Most architectures have these options on by default and not user selectable. Most architectures have these options on by default and not user selectable.
For some architectures like arm that wish to have these be selectable, For some architectures like arm that wish to have these be selectable,
the architecture Kconfig can select ARCH_OPTIONAL_KERNEL_RWX to enable 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. 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 Vast areas of kernel memory contain function pointers that are looked
up by the kernel and used to continue execution (e.g. descriptor/vector 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 ...@@ -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 of the kernel, gaining the protection of the kernel's strict memory
permissions as described above. permissions as described above.
For variables that are initialized once at __init time, these can For variables that are initialized once at ``__init`` time, these can
be marked with the (new and under development) __ro_after_init be marked with the (new and under development) ``__ro_after_init``
attribute. attribute.
What remains are variables that are updated rarely (e.g. GDT). These 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 ...@@ -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 CPU thread performing the update would be given uninterruptible write
access to the memory.) 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 The kernel must never execute userspace memory. The kernel must also never
access userspace memory without explicit expectation to do so. These 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 ...@@ -95,10 +102,11 @@ By blocking userspace memory in this way, execution and data parsing
cannot be passed to trivially-controlled userspace memory, forcing cannot be passed to trivially-controlled userspace memory, forcing
attacks to operate entirely in kernel memory. 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 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 The "seccomp" system provides an opt-in feature made available to
userspace, which provides a way to reduce the number of kernel entry 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 ...@@ -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 restricted to the more regular set of normally available to unprivileged
userspace. userspace.
### Restricting access to kernel modules Restricting access to kernel modules
------------------------------------
The kernel should never allow an unprivileged user the ability to The kernel should never allow an unprivileged user the ability to
load specific kernel modules, since that would provide a facility to load specific kernel modules, since that would provide a facility to
...@@ -127,11 +136,12 @@ for debate in some scenarios.) ...@@ -127,11 +136,12 @@ for debate in some scenarios.)
To protect against even privileged users, systems may need to either To protect against even privileged users, systems may need to either
disable module loading entirely (e.g. monolithic kernel builds or disable module loading entirely (e.g. monolithic kernel builds or
modules_disabled sysctl), or provide signed modules (e.g. 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. 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 There are many memory structures in the kernel that are regularly abused
to gain execution control during an attack, By far the most commonly 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 ...@@ -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 address stored on the stack is overwritten. Many other examples of this
kind of attack exist, and protections exist to defend against them. 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 The classic stack buffer overflow involves writing past the expected end
of a variable stored on the stack, ultimately writing a controlled value 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 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 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. 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 A less well understood attack is using a bug that triggers the
kernel to consume stack memory with deep function calls or large stack 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 ...@@ -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 sensitive thread_info structure elsewhere, and adding a faulting memory
hole at the bottom of the stack to catch these overflows. 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 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 allocation and freeing to make sure they aren't being used to manipulate
other memory areas. other memory areas.
### Counter integrity Counter integrity
-----------------
Many places in the kernel use atomic counters to track object references Many places in the kernel use atomic counters to track object references
or perform similar lifetime management. When these counters can be made or perform similar lifetime management. When these counters can be made
to wrap (over or under) this traditionally exposes a use-after-free to wrap (over or under) this traditionally exposes a use-after-free
flaw. By trapping atomic wrapping, this class of bug vanishes. 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) Similar to counter overflow, integer overflows (usually size calculations)
need to be detected at runtime to kill this class of bug, which 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. 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 While many protections can be considered deterministic (e.g. read-only
memory cannot be written to), some protections provide only statistical 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 ...@@ -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 running system to overcome the defense. While not perfect, these do
provide meaningful defenses. 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 It should be noted that things like the stack canary discussed earlier
are technically statistical defenses, since they rely on a secret value, 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. ...@@ -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 different canary per stack) and high entropy (e.g. is the RNG actually
working?) in order to maximize their success. 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 Since the location of kernel memory is almost always instrumental in
mounting a successful attack, making the location non-deterministic 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 ...@@ -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 the value of information exposures higher, since they may be used to
discover desired memory locations.) discover desired memory locations.)
#### Text and module base Text and module base
~~~~~~~~~~~~~~~~~~~~
By relocating the physical and virtual base address of the kernel at 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 frustrated. Additionally, offsetting the module loading base address
means that even systems that load the same set of modules in the same 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 order every boot will not share a common base address with the rest of
the kernel text. the kernel text.
#### Stack base Stack base
~~~~~~~~~~
If the base address of the kernel stack is not the same between processes, 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 or even not the same between syscalls, targets on or beyond the stack
become more difficult to locate. 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 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 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 ...@@ -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 between boots, targeting them is frustrated, requiring an information
exposure specific to the region. exposure specific to the region.
#### Structure layout Structure layout
~~~~~~~~~~~~~~~~
By performing a per-build randomization of the layout of sensitive By performing a per-build randomization of the layout of sensitive
structures, attacks must either be tuned to known kernel builds or expose 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 ...@@ -240,26 +262,30 @@ enough kernel memory to determine structure layouts before manipulating
them. them.
## Preventing Information Exposures Preventing Information Exposures
================================
Since the locations of sensitive structures are the primary target for Since the locations of sensitive structures are the primary target for
attacks, it is important to defend against exposure of both kernel memory attacks, it is important to defend against exposure of both kernel memory
addresses and kernel memory contents (since they may contain kernel addresses and kernel memory contents (since they may contain kernel
addresses or other sensitive things like canary values). addresses or other sensitive things like canary values).
### Unique identifiers Unique identifiers
------------------
Kernel memory addresses must never be used as identifiers exposed to Kernel memory addresses must never be used as identifiers exposed to
userspace. Instead, use an atomic counter, an idr, or similar unique userspace. Instead, use an atomic counter, an idr, or similar unique
identifier. identifier.
### Memory initialization Memory initialization
---------------------
Memory copied to userspace must always be fully initialized. If not Memory copied to userspace must always be fully initialized. If not
explicitly memset(), this will require changes to the compiler to make explicitly memset(), this will require changes to the compiler to make
sure structure holes are cleared. sure structure holes are cleared.
### Memory poisoning Memory poisoning
----------------
When releasing memory, it is best to poison the contents (clear stack on 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 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 ...@@ -267,9 +293,10 @@ rely on the old contents of memory. This frustrates many uninitialized
variable attacks, stack content exposures, heap content exposures, and variable attacks, stack content exposures, heap content exposures, and
use-after-free attacks. use-after-free attacks.
### Destination tracking Destination tracking
--------------------
To help kill classes of bugs that result in kernel addresses being To help kill classes of bugs that result in kernel addresses being
written to userspace, the destination of writes needs to be tracked. If 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. it should automatically censor sensitive values.
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