1 /* SPDX-License-Identifier: GPL-2.0-only */ 2 #ifndef _LINUX_RANDOMIZE_KSTACK_H 3 #define _LINUX_RANDOMIZE_KSTACK_H 4 5 #ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET 6 #include <linux/kernel.h> 7 #include <linux/jump_label.h> 8 #include <linux/percpu-defs.h> 9 10 DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, 11 randomize_kstack_offset); 12 DECLARE_PER_CPU(u32, kstack_offset); 13 14 /* 15 * Do not use this anywhere else in the kernel. This is used here because 16 * it provides an arch-agnostic way to grow the stack with correct 17 * alignment. Also, since this use is being explicitly masked to a max of 18 * 10 bits, stack-clash style attacks are unlikely. For more details see 19 * "VLAs" in Documentation/process/deprecated.rst 20 * 21 * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently 22 * only with Clang and not GCC). Initializing the unused area on each syscall 23 * entry is expensive, and generating an implicit call to memset() may also be 24 * problematic (such as in noinstr functions). Therefore, if the compiler 25 * supports it (which it should if it initializes allocas), always use the 26 * "uninitialized" variant of the builtin. 27 */ 28 #if __has_builtin(__builtin_alloca_uninitialized) 29 #define __kstack_alloca __builtin_alloca_uninitialized 30 #else 31 #define __kstack_alloca __builtin_alloca 32 #endif 33 34 /* 35 * Use, at most, 6 bits of entropy (on 64-bit; 8 on 32-bit). This cap is 36 * to keep the "VLA" from being unbounded (see above). Additionally clear 37 * the bottom 4 bits (on 64-bit systems, 2 for 32-bit), since stack 38 * alignment will always be at least word size. This makes the compiler 39 * code gen better when it is applying the actual per-arch alignment to 40 * the final offset. The resulting randomness is reasonable without overly 41 * constraining usable stack space. 42 */ 43 #ifdef CONFIG_64BIT 44 #define KSTACK_OFFSET_MAX(x) ((x) & 0b1111110000) 45 #else 46 #define KSTACK_OFFSET_MAX(x) ((x) & 0b1111111100) 47 #endif 48 49 /** 50 * add_random_kstack_offset - Increase stack utilization by previously 51 * chosen random offset 52 * 53 * This should be used in the syscall entry path when interrupts and 54 * preempt are disabled, and after user registers have been stored to 55 * the stack. For testing the resulting entropy, please see: 56 * tools/testing/selftests/lkdtm/stack-entropy.sh 57 */ 58 #define add_random_kstack_offset() do { \ 59 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ 60 &randomize_kstack_offset)) { \ 61 u32 offset = raw_cpu_read(kstack_offset); \ 62 u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ 63 /* Keep allocation even after "ptr" loses scope. */ \ 64 asm volatile("" :: "r"(ptr) : "memory"); \ 65 } \ 66 } while (0) 67 68 /** 69 * choose_random_kstack_offset - Choose the random offset for the next 70 * add_random_kstack_offset() 71 * 72 * This should only be used during syscall exit when interrupts and 73 * preempt are disabled. This position in the syscall flow is done to 74 * frustrate attacks from userspace attempting to learn the next offset: 75 * - Maximize the timing uncertainty visible from userspace: if the 76 * offset is chosen at syscall entry, userspace has much more control 77 * over the timing between choosing offsets. "How long will we be in 78 * kernel mode?" tends to be more difficult to predict than "how long 79 * will we be in user mode?" 80 * - Reduce the lifetime of the new offset sitting in memory during 81 * kernel mode execution. Exposure of "thread-local" memory content 82 * (e.g. current, percpu, etc) tends to be easier than arbitrary 83 * location memory exposure. 84 */ 85 #define choose_random_kstack_offset(rand) do { \ 86 if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ 87 &randomize_kstack_offset)) { \ 88 u32 offset = raw_cpu_read(kstack_offset); \ 89 offset = ror32(offset, 5) ^ (rand); \ 90 raw_cpu_write(kstack_offset, offset); \ 91 } \ 92 } while (0) 93 #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ 94 #define add_random_kstack_offset() do { } while (0) 95 #define choose_random_kstack_offset(rand) do { } while (0) 96 #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ 97 98 #endif 99