1 /* SPDX-License-Identifier: GPL-2.0-only */ 2 #ifndef _LINUX_RANDOMIZE_KSTACK_H 3 #define _LINUX_RANDOMIZE_KSTACK_H 4 5 #include <linux/kernel.h> 6 #include <linux/jump_label.h> 7 #include <linux/percpu-defs.h> 8 9 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, 10 randomize_kstack_offset); 11 DECLARE_PER_CPU(u32, kstack_offset); 12 13 /* 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 19 */ 20 void *__builtin_alloca(size_t size); 21 /* 22 * Use, at most, 10 bits of entropy. We explicitly cap this to keep the 23 * "VLA" from being unbounded (see above). 10 bits leaves enough room for 24 * per-arch offset masks to reduce entropy (by removing higher bits, since 25 * high entropy may overly constrain usable stack space), and for 26 * compiler/arch-specific stack alignment to remove the lower bits. 27 */ 28 #define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF) 29 30 /* 31 * These macros must be used during syscall entry when interrupts and 32 * preempt are disabled, and after user registers have been stored to 33 * the stack. 34 */ 35 #define add_random_kstack_offset() do { \ 36 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ 37 &randomize_kstack_offset)) { \ 38 u32 offset = raw_cpu_read(kstack_offset); \ 39 u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \ 40 /* Keep allocation even after "ptr" loses scope. */ \ 41 asm volatile("" :: "r"(ptr) : "memory"); \ 42 } \ 43 } while (0) 44 45 #define choose_random_kstack_offset(rand) do { \ 46 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ 47 &randomize_kstack_offset)) { \ 48 u32 offset = raw_cpu_read(kstack_offset); \ 49 offset ^= (rand); \ 50 raw_cpu_write(kstack_offset, offset); \ 51 } \ 52 } while (0) 53 54 #endif 55