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 58cb37a59SMarco 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 20efa90c11SMarco Elver * 21efa90c11SMarco Elver * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently 22efa90c11SMarco Elver * only with Clang and not GCC). Initializing the unused area on each syscall 23efa90c11SMarco Elver * entry is expensive, and generating an implicit call to memset() may also be 24efa90c11SMarco Elver * problematic (such as in noinstr functions). Therefore, if the compiler 25efa90c11SMarco Elver * supports it (which it should if it initializes allocas), always use the 26efa90c11SMarco Elver * "uninitialized" variant of the builtin. 2739218ff4SKees Cook */ 28efa90c11SMarco Elver #if __has_builtin(__builtin_alloca_uninitialized) 29efa90c11SMarco Elver #define __kstack_alloca __builtin_alloca_uninitialized 30efa90c11SMarco Elver #else 31efa90c11SMarco Elver #define __kstack_alloca __builtin_alloca 32efa90c11SMarco Elver #endif 33efa90c11SMarco Elver 3439218ff4SKees Cook /* 3539218ff4SKees Cook * Use, at most, 10 bits of entropy. We explicitly cap this to keep the 3639218ff4SKees Cook * "VLA" from being unbounded (see above). 10 bits leaves enough room for 3739218ff4SKees Cook * per-arch offset masks to reduce entropy (by removing higher bits, since 3839218ff4SKees Cook * high entropy may overly constrain usable stack space), and for 3939218ff4SKees Cook * compiler/arch-specific stack alignment to remove the lower bits. 4039218ff4SKees Cook */ 4139218ff4SKees Cook #define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF) 4239218ff4SKees Cook 43*1ff29758SKees Cook /** 44*1ff29758SKees Cook * add_random_kstack_offset - Increase stack utilization by previously 45*1ff29758SKees Cook * chosen random offset 46*1ff29758SKees Cook * 47*1ff29758SKees Cook * This should be used in the syscall entry path when interrupts and 4839218ff4SKees Cook * preempt are disabled, and after user registers have been stored to 49*1ff29758SKees Cook * the stack. For testing the resulting entropy, please see: 50*1ff29758SKees Cook * tools/testing/selftests/lkdtm/stack-entropy.sh 5139218ff4SKees Cook */ 5239218ff4SKees Cook #define add_random_kstack_offset() do { \ 5339218ff4SKees Cook if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ 5439218ff4SKees Cook &randomize_kstack_offset)) { \ 5539218ff4SKees Cook u32 offset = raw_cpu_read(kstack_offset); \ 56efa90c11SMarco Elver u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ 5739218ff4SKees Cook /* Keep allocation even after "ptr" loses scope. */ \ 582515dd6cSNick Desaulniers asm volatile("" :: "r"(ptr) : "memory"); \ 5939218ff4SKees Cook } \ 6039218ff4SKees Cook } while (0) 6139218ff4SKees Cook 62*1ff29758SKees Cook /** 63*1ff29758SKees Cook * choose_random_kstack_offset - Choose the random offset for the next 64*1ff29758SKees Cook * add_random_kstack_offset() 65*1ff29758SKees Cook * 66*1ff29758SKees Cook * This should only be used during syscall exit when interrupts and 67*1ff29758SKees Cook * preempt are disabled. This position in the syscall flow is done to 68*1ff29758SKees Cook * frustrate attacks from userspace attempting to learn the next offset: 69*1ff29758SKees Cook * - Maximize the timing uncertainty visible from userspace: if the 70*1ff29758SKees Cook * offset is chosen at syscall entry, userspace has much more control 71*1ff29758SKees Cook * over the timing between choosing offsets. "How long will we be in 72*1ff29758SKees Cook * kernel mode?" tends to be more difficult to predict than "how long 73*1ff29758SKees Cook * will we be in user mode?" 74*1ff29758SKees Cook * - Reduce the lifetime of the new offset sitting in memory during 75*1ff29758SKees Cook * kernel mode execution. Exposure of "thread-local" memory content 76*1ff29758SKees Cook * (e.g. current, percpu, etc) tends to be easier than arbitrary 77*1ff29758SKees Cook * location memory exposure. 78*1ff29758SKees Cook */ 7939218ff4SKees Cook #define choose_random_kstack_offset(rand) do { \ 8039218ff4SKees Cook if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ 8139218ff4SKees Cook &randomize_kstack_offset)) { \ 8239218ff4SKees Cook u32 offset = raw_cpu_read(kstack_offset); \ 8339218ff4SKees Cook offset ^= (rand); \ 8439218ff4SKees Cook raw_cpu_write(kstack_offset, offset); \ 8539218ff4SKees Cook } \ 8639218ff4SKees Cook } while (0) 878cb37a59SMarco Elver #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ 888cb37a59SMarco Elver #define add_random_kstack_offset() do { } while (0) 898cb37a59SMarco Elver #define choose_random_kstack_offset(rand) do { } while (0) 908cb37a59SMarco Elver #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ 9139218ff4SKees Cook 9239218ff4SKees Cook #endif 93