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 * 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. 26 */ 27 #if __has_builtin(__builtin_alloca_uninitialized) 28 #define __kstack_alloca __builtin_alloca_uninitialized 29 #else 30 #define __kstack_alloca __builtin_alloca 31 #endif 32 33 /* 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. 39 */ 40 #define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF) 41 42 /* 43 * These macros must be used during syscall entry when interrupts and 44 * preempt are disabled, and after user registers have been stored to 45 * the stack. 46 */ 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"); \ 54 } \ 55 } while (0) 56 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); \ 61 offset ^= (rand); \ 62 raw_cpu_write(kstack_offset, offset); \ 63 } \ 64 } while (0) 65 66 #endif 67