1 /* SPDX-License-Identifier: GPL-2.0-only */
2 #ifndef _LINUX_RANDOMIZE_KSTACK_H
3 #define _LINUX_RANDOMIZE_KSTACK_H
5 #include <linux/kernel.h>
6 #include <linux/jump_label.h>
7 #include <linux/percpu-defs.h>
9 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
10 randomize_kstack_offset);
11 DECLARE_PER_CPU(u32, kstack_offset);
14 * Do not use this anywhere else in the kernel. This is used here because
15 * it provides an arch-agnostic way to grow the stack with correct
16 * alignment. Also, since this use is being explicitly masked to a max of
17 * 10 bits, stack-clash style attacks are unlikely. For more details see
18 * "VLAs" in Documentation/process/deprecated.rst
20 * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently
21 * only with Clang and not GCC). Initializing the unused area on each syscall
22 * entry is expensive, and generating an implicit call to memset() may also be
23 * problematic (such as in noinstr functions). Therefore, if the compiler
24 * supports it (which it should if it initializes allocas), always use the
25 * "uninitialized" variant of the builtin.
27 #if __has_builtin(__builtin_alloca_uninitialized)
28 #define __kstack_alloca __builtin_alloca_uninitialized
30 #define __kstack_alloca __builtin_alloca
34 * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
35 * "VLA" from being unbounded (see above). 10 bits leaves enough room for
36 * per-arch offset masks to reduce entropy (by removing higher bits, since
37 * high entropy may overly constrain usable stack space), and for
38 * compiler/arch-specific stack alignment to remove the lower bits.
40 #define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF)
43 * These macros must be used during syscall entry when interrupts and
44 * preempt are disabled, and after user registers have been stored to
47 #define add_random_kstack_offset() do { \
48 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
49 &randomize_kstack_offset)) { \
50 u32 offset = raw_cpu_read(kstack_offset); \
51 u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \
52 /* Keep allocation even after "ptr" loses scope. */ \
53 asm volatile("" :: "r"(ptr) : "memory"); \
57 #define choose_random_kstack_offset(rand) do { \
58 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
59 &randomize_kstack_offset)) { \
60 u32 offset = raw_cpu_read(kstack_offset); \
62 raw_cpu_write(kstack_offset, offset); \