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1 | /* SPDX-License-Identifier: GPL-2.0-only */ |
2 | #ifndef _LINUX_RANDOMIZE_KSTACK_H | |
3 | #define _LINUX_RANDOMIZE_KSTACK_H | |
4 | ||
8cb37a59 | 5 | #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET |
39218ff4 KC |
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_KSTACK_OFFSET_DEFAULT, | |
11 | randomize_kstack_offset); | |
12 | DECLARE_PER_CPU(u32, 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 | |
efa90c11 ME |
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. | |
39218ff4 | 27 | */ |
efa90c11 ME |
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 | ||
39218ff4 KC |
34 | /* |
35 | * Use, at most, 10 bits of entropy. We explicitly cap this to keep the | |
36 | * "VLA" from being unbounded (see above). 10 bits leaves enough room for | |
37 | * per-arch offset masks to reduce entropy (by removing higher bits, since | |
38 | * high entropy may overly constrain usable stack space), and for | |
39 | * compiler/arch-specific stack alignment to remove the lower bits. | |
40 | */ | |
41 | #define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF) | |
42 | ||
1ff29758 KC |
43 | /** |
44 | * add_random_kstack_offset - Increase stack utilization by previously | |
45 | * chosen random offset | |
46 | * | |
47 | * This should be used in the syscall entry path when interrupts and | |
39218ff4 | 48 | * preempt are disabled, and after user registers have been stored to |
1ff29758 KC |
49 | * the stack. For testing the resulting entropy, please see: |
50 | * tools/testing/selftests/lkdtm/stack-entropy.sh | |
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51 | */ |
52 | #define add_random_kstack_offset() do { \ | |
53 | if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ | |
54 | &randomize_kstack_offset)) { \ | |
55 | u32 offset = raw_cpu_read(kstack_offset); \ | |
efa90c11 | 56 | u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ |
39218ff4 | 57 | /* Keep allocation even after "ptr" loses scope. */ \ |
2515dd6c | 58 | asm volatile("" :: "r"(ptr) : "memory"); \ |
39218ff4 KC |
59 | } \ |
60 | } while (0) | |
61 | ||
1ff29758 KC |
62 | /** |
63 | * choose_random_kstack_offset - Choose the random offset for the next | |
64 | * add_random_kstack_offset() | |
65 | * | |
66 | * This should only be used during syscall exit when interrupts and | |
67 | * preempt are disabled. This position in the syscall flow is done to | |
68 | * frustrate attacks from userspace attempting to learn the next offset: | |
69 | * - Maximize the timing uncertainty visible from userspace: if the | |
70 | * offset is chosen at syscall entry, userspace has much more control | |
71 | * over the timing between choosing offsets. "How long will we be in | |
72 | * kernel mode?" tends to be more difficult to predict than "how long | |
73 | * will we be in user mode?" | |
74 | * - Reduce the lifetime of the new offset sitting in memory during | |
75 | * kernel mode execution. Exposure of "thread-local" memory content | |
76 | * (e.g. current, percpu, etc) tends to be easier than arbitrary | |
77 | * location memory exposure. | |
78 | */ | |
39218ff4 KC |
79 | #define choose_random_kstack_offset(rand) do { \ |
80 | if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ | |
81 | &randomize_kstack_offset)) { \ | |
82 | u32 offset = raw_cpu_read(kstack_offset); \ | |
83 | offset ^= (rand); \ | |
84 | raw_cpu_write(kstack_offset, offset); \ | |
85 | } \ | |
86 | } while (0) | |
8cb37a59 ME |
87 | #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ |
88 | #define add_random_kstack_offset() do { } while (0) | |
89 | #define choose_random_kstack_offset(rand) do { } while (0) | |
90 | #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ | |
39218ff4 KC |
91 | |
92 | #endif |