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Ryan Roberts
a96ef5848c
Previously different architectures were using random sources of
differing strength and cost to decide the random kstack offset. A number
of architectures (loongarch, powerpc, s390, x86) were using their
timestamp counter, at whatever the frequency happened to be. Other
arches (arm64, riscv) were using entropy from the crng via
get_random_u16().
There have been concerns that in some cases the timestamp counters may
be too weak, because they can be easily guessed or influenced by user
space. And get_random_u16() has been shown to be too costly for the
level of protection kstack offset randomization provides.
So let's use a common, architecture-agnostic source of entropy; a
per-cpu prng, seeded at boot-time from the crng. This has a few
benefits:
- We can remove choose_random_kstack_offset(); That was only there to
try to make the timestamp counter value a bit harder to influence
from user space [*].
- The architecture code is simplified. All it has to do now is call
add_random_kstack_offset() in the syscall path.
- The strength of the randomness can be reasoned about independently
of the architecture.
- Arches previously using get_random_u16() now have much faster
syscall paths, see below results.
[*] Additionally, this gets rid of some redundant work on s390 and x86.
Before this patch, those architectures called
choose_random_kstack_offset() under arch_exit_to_user_mode_prepare(),
which is also called for exception returns to userspace which were *not*
syscalls (e.g. regular interrupts). Getting rid of
choose_random_kstack_offset() avoids a small amount of redundant work
for the non-syscall cases.
In some configurations, add_random_kstack_offset() will now call
instrumentable code, so for a couple of arches, I have moved the call a
bit later to the first point where instrumentation is allowed. This
doesn't impact the efficacy of the mechanism.
There have been some claims that a prng may be less strong than the
timestamp counter if not regularly reseeded. But the prng has a period
of about 2^113. So as long as the prng state remains secret, it should
not be possible to guess. If the prng state can be accessed, we have
bigger problems.
Additionally, we are only consuming 6 bits to randomize the stack, so
there are only 64 possible random offsets. I assert that it would be
trivial for an attacker to brute force by repeating their attack and
waiting for the random stack offset to be the desired one. The prng
approach seems entirely proportional to this level of protection.
Performance data are provided below. The baseline is v6.18 with rndstack
on for each respective arch. (I)/(R) indicate statistically significant
improvement/regression. arm64 platform is AWS Graviton3 (m7g.metal).
x86_64 platform is AWS Sapphire Rapids (m7i.24xlarge):
+-----------------+--------------+---------------+---------------+
| Benchmark | Result Class | per-cpu-prng | per-cpu-prng |
| | | arm64 (metal) | x86_64 (VM) |
+=================+==============+===============+===============+
| syscall/getpid | mean (ns) | (I) -9.50% | (I) -17.65% |
| | p99 (ns) | (I) -59.24% | (I) -24.41% |
| | p99.9 (ns) | (I) -59.52% | (I) -28.52% |
+-----------------+--------------+---------------+---------------+
| syscall/getppid | mean (ns) | (I) -9.52% | (I) -19.24% |
| | p99 (ns) | (I) -59.25% | (I) -25.03% |
| | p99.9 (ns) | (I) -59.50% | (I) -28.17% |
+-----------------+--------------+---------------+---------------+
| syscall/invalid | mean (ns) | (I) -10.31% | (I) -18.56% |
| | p99 (ns) | (I) -60.79% | (I) -20.06% |
| | p99.9 (ns) | (I) -61.04% | (I) -25.04% |
+-----------------+--------------+---------------+---------------+
I tested an earlier version of this change on x86 bare metal and it
showed a smaller but still significant improvement. The bare metal
system wasn't available this time around so testing was done in a VM
instance. I'm guessing the cost of rdtsc is higher for VMs.
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://patch.msgid.link/20260303150840.3789438-3-ryan.roberts@arm.com
Signed-off-by: Kees Cook <kees@kernel.org>
85 lines
2.9 KiB
C
85 lines
2.9 KiB
C
/* SPDX-License-Identifier: GPL-2.0-only */
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#ifndef _LINUX_RANDOMIZE_KSTACK_H
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#define _LINUX_RANDOMIZE_KSTACK_H
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#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
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#include <linux/kernel.h>
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#include <linux/jump_label.h>
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#include <linux/percpu-defs.h>
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#include <linux/prandom.h>
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DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
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randomize_kstack_offset);
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/*
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* Do not use this anywhere else in the kernel. This is used here because
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* it provides an arch-agnostic way to grow the stack with correct
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* alignment. Also, since this use is being explicitly masked to a max of
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* 10 bits, stack-clash style attacks are unlikely. For more details see
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* "VLAs" in Documentation/process/deprecated.rst
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*
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* The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
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* only with Clang and not GCC). Initializing the unused area on each syscall
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* entry is expensive, and generating an implicit call to memset() may also be
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* problematic (such as in noinstr functions). Therefore, if the compiler
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* supports it (which it should if it initializes allocas), always use the
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* "uninitialized" variant of the builtin.
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*/
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#if __has_builtin(__builtin_alloca_uninitialized)
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#define __kstack_alloca __builtin_alloca_uninitialized
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#else
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#define __kstack_alloca __builtin_alloca
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#endif
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/*
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* Use, at most, 6 bits of entropy (on 64-bit; 8 on 32-bit). This cap is
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* to keep the "VLA" from being unbounded (see above). Additionally clear
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* the bottom 4 bits (on 64-bit systems, 2 for 32-bit), since stack
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* alignment will always be at least word size. This makes the compiler
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* code gen better when it is applying the actual per-arch alignment to
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* the final offset. The resulting randomness is reasonable without overly
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* constraining usable stack space.
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*/
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#ifdef CONFIG_64BIT
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#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111110000)
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#else
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#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111111100)
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#endif
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DECLARE_PER_CPU(struct rnd_state, kstack_rnd_state);
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static __always_inline u32 get_kstack_offset(void)
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{
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struct rnd_state *state;
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u32 rnd;
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state = &get_cpu_var(kstack_rnd_state);
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rnd = prandom_u32_state(state);
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put_cpu_var(kstack_rnd_state);
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return rnd;
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}
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/**
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* add_random_kstack_offset - Increase stack utilization by a random offset.
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*
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* This should be used in the syscall entry path after user registers have been
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* stored to the stack. Preemption may be enabled. For testing the resulting
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* entropy, please see: tools/testing/selftests/lkdtm/stack-entropy.sh
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*/
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#define add_random_kstack_offset() do { \
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if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
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&randomize_kstack_offset)) { \
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u32 offset = get_kstack_offset(); \
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u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \
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/* Keep allocation even after "ptr" loses scope. */ \
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asm volatile("" :: "r"(ptr) : "memory"); \
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} \
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} while (0)
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#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
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#define add_random_kstack_offset() do { } while (0)
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#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
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#endif
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