Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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linux
1/* SPDX-License-Identifier: GPL-2.0-only */
2#ifndef _LINUX_RANDOMIZE_KSTACK_H
3#define _LINUX_RANDOMIZE_KSTACK_H
4
5#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
6#include <linux/kernel.h>
7#include <linux/jump_label.h>
8#include <linux/percpu-defs.h>
9#include <linux/prandom.h>
10
11DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
12 randomize_kstack_offset);
13
14/*
15 * Do not use this anywhere else in the kernel. This is used here because
16 * it provides an arch-agnostic way to grow the stack with correct
17 * alignment. Also, since this use is being explicitly masked to a max of
18 * 10 bits, stack-clash style attacks are unlikely. For more details see
19 * "VLAs" in Documentation/process/deprecated.rst
20 *
21 * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
22 * only with Clang and not GCC). Initializing the unused area on each syscall
23 * entry is expensive, and generating an implicit call to memset() may also be
24 * problematic (such as in noinstr functions). Therefore, if the compiler
25 * supports it (which it should if it initializes allocas), always use the
26 * "uninitialized" variant of the builtin.
27 */
28#if __has_builtin(__builtin_alloca_uninitialized)
29#define __kstack_alloca __builtin_alloca_uninitialized
30#else
31#define __kstack_alloca __builtin_alloca
32#endif
33
34/*
35 * Use, at most, 6 bits of entropy (on 64-bit; 8 on 32-bit). This cap is
36 * to keep the "VLA" from being unbounded (see above). Additionally clear
37 * the bottom 4 bits (on 64-bit systems, 2 for 32-bit), since stack
38 * alignment will always be at least word size. This makes the compiler
39 * code gen better when it is applying the actual per-arch alignment to
40 * the final offset. The resulting randomness is reasonable without overly
41 * constraining usable stack space.
42 */
43#ifdef CONFIG_64BIT
44#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111110000)
45#else
46#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111111100)
47#endif
48
49DECLARE_PER_CPU(struct rnd_state, kstack_rnd_state);
50
51static __always_inline u32 get_kstack_offset(void)
52{
53 struct rnd_state *state;
54 u32 rnd;
55
56 state = &get_cpu_var(kstack_rnd_state);
57 rnd = prandom_u32_state(state);
58 put_cpu_var(kstack_rnd_state);
59
60 return rnd;
61}
62
63/**
64 * add_random_kstack_offset - Increase stack utilization by a random offset.
65 *
66 * This should be used in the syscall entry path after user registers have been
67 * stored to the stack. Preemption may be enabled. For testing the resulting
68 * entropy, please see: tools/testing/selftests/lkdtm/stack-entropy.sh
69 */
70#define add_random_kstack_offset() do { \
71 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
72 &randomize_kstack_offset)) { \
73 u32 offset = get_kstack_offset(); \
74 u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \
75 /* Keep allocation even after "ptr" loses scope. */ \
76 asm volatile("" :: "r"(ptr) : "memory"); \
77 } \
78} while (0)
79
80#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
81#define add_random_kstack_offset() do { } while (0)
82#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
83
84#endif