139218ff4SKees Cook /* SPDX-License-Identifier: GPL-2.0-only */ 239218ff4SKees Cook #ifndef _LINUX_RANDOMIZE_KSTACK_H 339218ff4SKees Cook #define _LINUX_RANDOMIZE_KSTACK_H 439218ff4SKees Cook 5*8cb37a59SMarco Elver #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET 639218ff4SKees Cook #include <linux/kernel.h> 739218ff4SKees Cook #include <linux/jump_label.h> 839218ff4SKees Cook #include <linux/percpu-defs.h> 939218ff4SKees Cook 1039218ff4SKees Cook DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, 1139218ff4SKees Cook randomize_kstack_offset); 1239218ff4SKees Cook DECLARE_PER_CPU(u32, kstack_offset); 1339218ff4SKees Cook 1439218ff4SKees Cook /* 1539218ff4SKees Cook * Do not use this anywhere else in the kernel. This is used here because 1639218ff4SKees Cook * it provides an arch-agnostic way to grow the stack with correct 1739218ff4SKees Cook * alignment. Also, since this use is being explicitly masked to a max of 1839218ff4SKees Cook * 10 bits, stack-clash style attacks are unlikely. For more details see 1939218ff4SKees Cook * "VLAs" in Documentation/process/deprecated.rst 2039218ff4SKees Cook */ 2139218ff4SKees Cook void *__builtin_alloca(size_t size); 2239218ff4SKees Cook /* 2339218ff4SKees Cook * Use, at most, 10 bits of entropy. We explicitly cap this to keep the 2439218ff4SKees Cook * "VLA" from being unbounded (see above). 10 bits leaves enough room for 2539218ff4SKees Cook * per-arch offset masks to reduce entropy (by removing higher bits, since 2639218ff4SKees Cook * high entropy may overly constrain usable stack space), and for 2739218ff4SKees Cook * compiler/arch-specific stack alignment to remove the lower bits. 2839218ff4SKees Cook */ 2939218ff4SKees Cook #define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF) 3039218ff4SKees Cook 3139218ff4SKees Cook /* 3239218ff4SKees Cook * These macros must be used during syscall entry when interrupts and 3339218ff4SKees Cook * preempt are disabled, and after user registers have been stored to 3439218ff4SKees Cook * the stack. 3539218ff4SKees Cook */ 3639218ff4SKees Cook #define add_random_kstack_offset() do { \ 3739218ff4SKees Cook if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ 3839218ff4SKees Cook &randomize_kstack_offset)) { \ 3939218ff4SKees Cook u32 offset = raw_cpu_read(kstack_offset); \ 4039218ff4SKees Cook u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \ 4139218ff4SKees Cook /* Keep allocation even after "ptr" loses scope. */ \ 422515dd6cSNick Desaulniers asm volatile("" :: "r"(ptr) : "memory"); \ 4339218ff4SKees Cook } \ 4439218ff4SKees Cook } while (0) 4539218ff4SKees Cook 4639218ff4SKees Cook #define choose_random_kstack_offset(rand) do { \ 4739218ff4SKees Cook if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ 4839218ff4SKees Cook &randomize_kstack_offset)) { \ 4939218ff4SKees Cook u32 offset = raw_cpu_read(kstack_offset); \ 5039218ff4SKees Cook offset ^= (rand); \ 5139218ff4SKees Cook raw_cpu_write(kstack_offset, offset); \ 5239218ff4SKees Cook } \ 5339218ff4SKees Cook } while (0) 54*8cb37a59SMarco Elver #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ 55*8cb37a59SMarco Elver #define add_random_kstack_offset() do { } while (0) 56*8cb37a59SMarco Elver #define choose_random_kstack_offset(rand) do { } while (0) 57*8cb37a59SMarco Elver #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ 5839218ff4SKees Cook 5939218ff4SKees Cook #endif 60