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Mark Salter
2a1a9e3ac9
JIRA: https://redhat.atlassian.net/browse/RHEL-215975 Conflicts: arch/loongarch/kernel/syscall.c arch/x86/entry/syscall_32.c Drop these bits not used in RHEL. commit a96ef5848cb096226bf6aff31a90d8b136d99b71 Author: Ryan Roberts <ryan.roberts@arm.com> Date: Tue, 3 Mar 2026 15:08:39 +0000 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> Signed-off-by: Mark Salter <msalter@redhat.com>
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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