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
10DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
11 randomize_kstack_offset);
12DECLARE_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
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, 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
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
48 * preempt are disabled, and after user registers have been stored to
49 * the stack. For testing the resulting entropy, please see:
50 * tools/testing/selftests/lkdtm/stack-entropy.sh
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); \
56 u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \
57 /* Keep allocation even after "ptr" loses scope. */ \
58 asm volatile("" :: "r"(ptr) : "memory"); \
59 } \
60} while (0)
61
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 */
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)
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 */
91
92#endif
93

source code of linux/include/linux/randomize_kstack.h