1 /* SPDX-License-Identifier: GPL-2.0-only */ 1
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
10 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTA
11 randomize_kstack_offs
12 DECLARE_PER_CPU(u32, kstack_offset);
13
14 /*
15 * Do not use this anywhere else in the kernel
16 * it provides an arch-agnostic way to grow th
17 * alignment. Also, since this use is being ex
18 * 10 bits, stack-clash style attacks are unli
19 * "VLAs" in Documentation/process/deprecated.
20 *
21 * The normal __builtin_alloca() is initialize
22 * only with Clang and not GCC). Initializing
23 * entry is expensive, and generating an impli
24 * problematic (such as in noinstr functions).
25 * supports it (which it should if it initiali
26 * "uninitialized" variant of the builtin.
27 */
28 #if __has_builtin(__builtin_alloca_uninitializ
29 #define __kstack_alloca __builtin_alloca_unini
30 #else
31 #define __kstack_alloca __builtin_alloca
32 #endif
33
34 /*
35 * Use, at most, 6 bits of entropy (on 64-bit;
36 * to keep the "VLA" from being unbounded (see
37 * the bottom 4 bits (on 64-bit systems, 2 for
38 * alignment will always be at least word size
39 * code gen better when it is applying the act
40 * the final offset. The resulting randomness
41 * constraining usable stack space.
42 */
43 #ifdef CONFIG_64BIT
44 #define KSTACK_OFFSET_MAX(x) ((x) & 0b11111
45 #else
46 #define KSTACK_OFFSET_MAX(x) ((x) & 0b11111
47 #endif
48
49 /**
50 * add_random_kstack_offset - Increase stack u
51 * chosen random of
52 *
53 * This should be used in the syscall entry pa
54 * preempt are disabled, and after user regist
55 * the stack. For testing the resulting entrop
56 * tools/testing/selftests/lkdtm/stack-entropy
57 */
58 #define add_random_kstack_offset() do {
59 if (static_branch_maybe(CONFIG_RANDOMI
60 &randomize_kst
61 u32 offset = raw_cpu_read(ksta
62 u8 *ptr = __kstack_alloca(KSTA
63 /* Keep allocation even after
64 asm volatile("" :: "r"(ptr) :
65 }
66 } while (0)
67
68 /**
69 * choose_random_kstack_offset - Choose the ra
70 * add_random_ks
71 *
72 * This should only be used during syscall exi
73 * preempt are disabled. This position in the
74 * frustrate attacks from userspace attempting
75 * - Maximize the timing uncertainty visible f
76 * offset is chosen at syscall entry, usersp
77 * over the timing between choosing offsets.
78 * kernel mode?" tends to be more difficult
79 * will we be in user mode?"
80 * - Reduce the lifetime of the new offset sit
81 * kernel mode execution. Exposure of "threa
82 * (e.g. current, percpu, etc) tends to be e
83 * location memory exposure.
84 */
85 #define choose_random_kstack_offset(rand) do {
86 if (static_branch_maybe(CONFIG_RANDOMI
87 &randomize_kst
88 u32 offset = raw_cpu_read(ksta
89 offset = ror32(offset, 5) ^ (r
90 raw_cpu_write(kstack_offset, o
91 }
92 } while (0)
93 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
94 #define add_random_kstack_offset()
95 #define choose_random_kstack_offset(rand)
96 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
97
98 #endif
99
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