1 /* SPDX-License-Identifier: GPL-2.0-only */ 1 /* SPDX-License-Identifier: GPL-2.0-only */
2 #ifndef _LINUX_RANDOMIZE_KSTACK_H 2 #ifndef _LINUX_RANDOMIZE_KSTACK_H
3 #define _LINUX_RANDOMIZE_KSTACK_H 3 #define _LINUX_RANDOMIZE_KSTACK_H
4 4
5 #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET 5 #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
6 #include <linux/kernel.h> 6 #include <linux/kernel.h>
7 #include <linux/jump_label.h> 7 #include <linux/jump_label.h>
8 #include <linux/percpu-defs.h> 8 #include <linux/percpu-defs.h>
9 9
10 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTA 10 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
11 randomize_kstack_offs 11 randomize_kstack_offset);
12 DECLARE_PER_CPU(u32, kstack_offset); 12 DECLARE_PER_CPU(u32, kstack_offset);
13 13
14 /* 14 /*
15 * Do not use this anywhere else in the kernel 15 * Do not use this anywhere else in the kernel. This is used here because
16 * it provides an arch-agnostic way to grow th 16 * it provides an arch-agnostic way to grow the stack with correct
17 * alignment. Also, since this use is being ex 17 * alignment. Also, since this use is being explicitly masked to a max of
18 * 10 bits, stack-clash style attacks are unli 18 * 10 bits, stack-clash style attacks are unlikely. For more details see
19 * "VLAs" in Documentation/process/deprecated. 19 * "VLAs" in Documentation/process/deprecated.rst
20 * 20 *
21 * The normal __builtin_alloca() is initialize 21 * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
22 * only with Clang and not GCC). Initializing 22 * only with Clang and not GCC). Initializing the unused area on each syscall
23 * entry is expensive, and generating an impli 23 * entry is expensive, and generating an implicit call to memset() may also be
24 * problematic (such as in noinstr functions). 24 * problematic (such as in noinstr functions). Therefore, if the compiler
25 * supports it (which it should if it initiali 25 * supports it (which it should if it initializes allocas), always use the
26 * "uninitialized" variant of the builtin. 26 * "uninitialized" variant of the builtin.
27 */ 27 */
28 #if __has_builtin(__builtin_alloca_uninitializ 28 #if __has_builtin(__builtin_alloca_uninitialized)
29 #define __kstack_alloca __builtin_alloca_unini 29 #define __kstack_alloca __builtin_alloca_uninitialized
30 #else 30 #else
31 #define __kstack_alloca __builtin_alloca 31 #define __kstack_alloca __builtin_alloca
32 #endif 32 #endif
33 33
34 /* 34 /*
35 * Use, at most, 6 bits of entropy (on 64-bit; !! 35 * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
36 * to keep the "VLA" from being unbounded (see !! 36 * "VLA" from being unbounded (see above). 10 bits leaves enough room for
37 * the bottom 4 bits (on 64-bit systems, 2 for !! 37 * per-arch offset masks to reduce entropy (by removing higher bits, since
38 * alignment will always be at least word size !! 38 * high entropy may overly constrain usable stack space), and for
39 * code gen better when it is applying the act !! 39 * compiler/arch-specific stack alignment to remove the lower bits.
40 * the final offset. The resulting randomness <<
41 * constraining usable stack space. <<
42 */ 40 */
43 #ifdef CONFIG_64BIT !! 41 #define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF)
44 #define KSTACK_OFFSET_MAX(x) ((x) & 0b11111 <<
45 #else <<
46 #define KSTACK_OFFSET_MAX(x) ((x) & 0b11111 <<
47 #endif <<
48 42
49 /** 43 /**
50 * add_random_kstack_offset - Increase stack u 44 * add_random_kstack_offset - Increase stack utilization by previously
51 * chosen random of 45 * chosen random offset
52 * 46 *
53 * This should be used in the syscall entry pa 47 * This should be used in the syscall entry path when interrupts and
54 * preempt are disabled, and after user regist 48 * preempt are disabled, and after user registers have been stored to
55 * the stack. For testing the resulting entrop 49 * the stack. For testing the resulting entropy, please see:
56 * tools/testing/selftests/lkdtm/stack-entropy 50 * tools/testing/selftests/lkdtm/stack-entropy.sh
57 */ 51 */
58 #define add_random_kstack_offset() do { 52 #define add_random_kstack_offset() do { \
59 if (static_branch_maybe(CONFIG_RANDOMI 53 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
60 &randomize_kst 54 &randomize_kstack_offset)) { \
61 u32 offset = raw_cpu_read(ksta 55 u32 offset = raw_cpu_read(kstack_offset); \
62 u8 *ptr = __kstack_alloca(KSTA 56 u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \
63 /* Keep allocation even after 57 /* Keep allocation even after "ptr" loses scope. */ \
64 asm volatile("" :: "r"(ptr) : 58 asm volatile("" :: "r"(ptr) : "memory"); \
65 } 59 } \
66 } while (0) 60 } while (0)
67 61
68 /** 62 /**
69 * choose_random_kstack_offset - Choose the ra 63 * choose_random_kstack_offset - Choose the random offset for the next
70 * add_random_ks 64 * add_random_kstack_offset()
71 * 65 *
72 * This should only be used during syscall exi 66 * This should only be used during syscall exit when interrupts and
73 * preempt are disabled. This position in the 67 * preempt are disabled. This position in the syscall flow is done to
74 * frustrate attacks from userspace attempting 68 * frustrate attacks from userspace attempting to learn the next offset:
75 * - Maximize the timing uncertainty visible f 69 * - Maximize the timing uncertainty visible from userspace: if the
76 * offset is chosen at syscall entry, usersp 70 * offset is chosen at syscall entry, userspace has much more control
77 * over the timing between choosing offsets. 71 * over the timing between choosing offsets. "How long will we be in
78 * kernel mode?" tends to be more difficult 72 * kernel mode?" tends to be more difficult to predict than "how long
79 * will we be in user mode?" 73 * will we be in user mode?"
80 * - Reduce the lifetime of the new offset sit 74 * - Reduce the lifetime of the new offset sitting in memory during
81 * kernel mode execution. Exposure of "threa 75 * kernel mode execution. Exposure of "thread-local" memory content
82 * (e.g. current, percpu, etc) tends to be e 76 * (e.g. current, percpu, etc) tends to be easier than arbitrary
83 * location memory exposure. 77 * location memory exposure.
84 */ 78 */
85 #define choose_random_kstack_offset(rand) do { 79 #define choose_random_kstack_offset(rand) do { \
86 if (static_branch_maybe(CONFIG_RANDOMI 80 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
87 &randomize_kst 81 &randomize_kstack_offset)) { \
88 u32 offset = raw_cpu_read(ksta 82 u32 offset = raw_cpu_read(kstack_offset); \
89 offset = ror32(offset, 5) ^ (r 83 offset = ror32(offset, 5) ^ (rand); \
90 raw_cpu_write(kstack_offset, o 84 raw_cpu_write(kstack_offset, offset); \
91 } 85 } \
92 } while (0) 86 } while (0)
93 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ 87 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
94 #define add_random_kstack_offset() 88 #define add_random_kstack_offset() do { } while (0)
95 #define choose_random_kstack_offset(rand) 89 #define choose_random_kstack_offset(rand) do { } while (0)
96 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ 90 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
97 91
98 #endif 92 #endif
99 93
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