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# SPDX-License-Identifier: GPL-2.0-only
menu "Kernel hardening options"
config GCC_PLUGIN_STRUCTLEAK
bool
help
While the kernel is built with warnings enabled for any missed
stack variable initializations, this warning is silenced for
anything passed by reference to another function, under the
occasionally misguided assumption that the function will do
the initialization. As this regularly leads to exploitable
flaws, this plugin is available to identify and zero-initialize
such variables, depending on the chosen level of coverage.
This plugin was originally ported from grsecurity/PaX. More
information at:
* https://grsecurity.net/
* https://pax.grsecurity.net/
menu "Memory initialization"
config CC_HAS_AUTO_VAR_INIT_PATTERN
def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
config CC_HAS_AUTO_VAR_INIT_ZERO
# GCC ignores the -enable flag, so we can test for the feature with
# a single invocation using the flag, but drop it as appropriate in
# the Makefile, depending on the presence of Clang.
def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
choice
prompt "Initialize kernel stack variables at function entry"
default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
default INIT_STACK_NONE
help
This option enables initialization of stack variables at
function entry time. This has the possibility to have the
greatest coverage (since all functions can have their
variables initialized), but the performance impact depends
on the function calling complexity of a given workload's
syscalls.
This chooses the level of coverage over classes of potentially
uninitialized variables. The selected class of variable will be
initialized before use in a function.
config INIT_STACK_NONE
bool "no automatic stack variable initialization (weakest)"
help
Disable automatic stack variable initialization.
This leaves the kernel vulnerable to the standard
classes of uninitialized stack variable exploits
and information exposures.
config GCC_PLUGIN_STRUCTLEAK_USER
bool "zero-init structs marked for userspace (weak)"
# Plugin can be removed once the kernel only supports GCC 12+
depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
select GCC_PLUGIN_STRUCTLEAK
help
Zero-initialize any structures on the stack containing
a __user attribute. This can prevent some classes of
uninitialized stack variable exploits and information
exposures, like CVE-2013-2141:
https://git.kernel.org/linus/b9e146d8eb3b9eca
config GCC_PLUGIN_STRUCTLEAK_BYREF
bool "zero-init structs passed by reference (strong)"
# Plugin can be removed once the kernel only supports GCC 12+
depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
depends on !(KASAN && KASAN_STACK)
select GCC_PLUGIN_STRUCTLEAK
help
Zero-initialize any structures on the stack that may
be passed by reference and had not already been
explicitly initialized. This can prevent most classes
of uninitialized stack variable exploits and information
exposures, like CVE-2017-1000410:
https://git.kernel.org/linus/06e7e776ca4d3654
As a side-effect, this keeps a lot of variables on the
stack that can otherwise be optimized out, so combining
this with CONFIG_KASAN_STACK can lead to a stack overflow
and is disallowed.
config GCC_PLUGIN_STRUCTLEAK_BYREF_ALL
bool "zero-init everything passed by reference (very strong)"
# Plugin can be removed once the kernel only supports GCC 12+
depends on GCC_PLUGINS && !CC_HAS_AUTO_VAR_INIT_ZERO
depends on !(KASAN && KASAN_STACK)
select GCC_PLUGIN_STRUCTLEAK
help
Zero-initialize any stack variables that may be passed
by reference and had not already been explicitly
initialized. This is intended to eliminate all classes
of uninitialized stack variable exploits and information
exposures.
As a side-effect, this keeps a lot of variables on the
stack that can otherwise be optimized out, so combining
this with CONFIG_KASAN_STACK can lead to a stack overflow
and is disallowed.
config INIT_STACK_ALL_PATTERN
bool "pattern-init everything (strongest)"
depends on CC_HAS_AUTO_VAR_INIT_PATTERN
help
Initializes everything on the stack (including padding)
with a specific debug value. This is intended to eliminate
all classes of uninitialized stack variable exploits and
information exposures, even variables that were warned about
having been left uninitialized.
Pattern initialization is known to provoke many existing bugs
related to uninitialized locals, e.g. pointers receive
non-NULL values, buffer sizes and indices are very big. The
pattern is situation-specific; Clang on 64-bit uses 0xAA
repeating for all types and padding except float and double
which use 0xFF repeating (-NaN). Clang on 32-bit uses 0xFF
repeating for all types and padding.
config INIT_STACK_ALL_ZERO
bool "zero-init everything (strongest and safest)"
depends on CC_HAS_AUTO_VAR_INIT_ZERO
help
Initializes everything on the stack (including padding)
with a zero value. This is intended to eliminate all
classes of uninitialized stack variable exploits and
information exposures, even variables that were warned
about having been left uninitialized.
Zero initialization provides safe defaults for strings
(immediately NUL-terminated), pointers (NULL), indices
(index 0), and sizes (0 length), so it is therefore more
suitable as a production security mitigation than pattern
initialization.
endchoice
config GCC_PLUGIN_STRUCTLEAK_VERBOSE
bool "Report forcefully initialized variables"
depends on GCC_PLUGIN_STRUCTLEAK
depends on !COMPILE_TEST # too noisy
help
This option will cause a warning to be printed each time the
structleak plugin finds a variable it thinks needs to be
initialized. Since not all existing initializers are detected
by the plugin, this can produce false positive warnings.
config GCC_PLUGIN_STACKLEAK
bool "Poison kernel stack before returning from syscalls"
depends on GCC_PLUGINS
depends on HAVE_ARCH_STACKLEAK
help
This option makes the kernel erase the kernel stack before
returning from system calls. This has the effect of leaving
the stack initialized to the poison value, which both reduces
the lifetime of any sensitive stack contents and reduces
potential for uninitialized stack variable exploits or information
exposures (it does not cover functions reaching the same stack
depth as prior functions during the same syscall). This blocks
most uninitialized stack variable attacks, with the performance
impact being driven by the depth of the stack usage, rather than
the function calling complexity.
The performance impact on a single CPU system kernel compilation
sees a 1% slowdown, other systems and workloads may vary and you
are advised to test this feature on your expected workload before
deploying it.
This plugin was ported from grsecurity/PaX. More information at:
* https://grsecurity.net/
* https://pax.grsecurity.net/
config STACKLEAK_TRACK_MIN_SIZE
int "Minimum stack frame size of functions tracked by STACKLEAK"
default 100
range 0 4096
depends on GCC_PLUGIN_STACKLEAK
help
The STACKLEAK gcc plugin instruments the kernel code for tracking
the lowest border of the kernel stack (and for some other purposes).
It inserts the stackleak_track_stack() call for the functions with
a stack frame size greater than or equal to this parameter.
If unsure, leave the default value 100.
config STACKLEAK_METRICS
bool "Show STACKLEAK metrics in the /proc file system"
depends on GCC_PLUGIN_STACKLEAK
depends on PROC_FS
help
If this is set, STACKLEAK metrics for every task are available in
the /proc file system. In particular, /proc/<pid>/stack_depth
shows the maximum kernel stack consumption for the current and
previous syscalls. Although this information is not precise, it
can be useful for estimating the STACKLEAK performance impact for
your workloads.
config STACKLEAK_RUNTIME_DISABLE
bool "Allow runtime disabling of kernel stack erasing"
depends on GCC_PLUGIN_STACKLEAK
help
This option provides 'stack_erasing' sysctl, which can be used in
runtime to control kernel stack erasing for kernels built with
CONFIG_GCC_PLUGIN_STACKLEAK.
config INIT_ON_ALLOC_DEFAULT_ON
bool "Enable heap memory zeroing on allocation by default"
help
This has the effect of setting "init_on_alloc=1" on the kernel
command line. This can be disabled with "init_on_alloc=0".
When "init_on_alloc" is enabled, all page allocator and slab
allocator memory will be zeroed when allocated, eliminating
many kinds of "uninitialized heap memory" flaws, especially
heap content exposures. The performance impact varies by
workload, but most cases see <1% impact. Some synthetic
workloads have measured as high as 7%.
config INIT_ON_FREE_DEFAULT_ON
bool "Enable heap memory zeroing on free by default"
help
This has the effect of setting "init_on_free=1" on the kernel
command line. This can be disabled with "init_on_free=0".
Similar to "init_on_alloc", when "init_on_free" is enabled,
all page allocator and slab allocator memory will be zeroed
when freed, eliminating many kinds of "uninitialized heap memory"
flaws, especially heap content exposures. The primary difference
with "init_on_free" is that data lifetime in memory is reduced,
as anything freed is wiped immediately, making live forensics or
cold boot memory attacks unable to recover freed memory contents.
The performance impact varies by workload, but is more expensive
than "init_on_alloc" due to the negative cache effects of
touching "cold" memory areas. Most cases see 3-5% impact. Some
synthetic workloads have measured as high as 8%.
config CC_HAS_ZERO_CALL_USED_REGS
def_bool $(cc-option,-fzero-call-used-regs=used-gpr)
config ZERO_CALL_USED_REGS
bool "Enable register zeroing on function exit"
depends on CC_HAS_ZERO_CALL_USED_REGS
help
At the end of functions, always zero any caller-used register
contents. This helps ensure that temporary values are not
leaked beyond the function boundary. This means that register
contents are less likely to be available for side channels
and information exposures. Additionally, this helps reduce the
number of useful ROP gadgets by about 20% (and removes compiler
generated "write-what-where" gadgets) in the resulting kernel
image. This has a less than 1% performance impact on most
workloads. Image size growth depends on architecture, and should
be evaluated for suitability. For example, x86_64 grows by less
than 1%, and arm64 grows by about 5%.
endmenu
endmenu
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