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Diffstat (limited to 'include/linux/randomize_kstack.h')
| -rw-r--r-- | include/linux/randomize_kstack.h | 98 |
1 files changed, 98 insertions, 0 deletions
diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h new file mode 100644 index 000000000000..1d982dbdd0d0 --- /dev/null +++ b/include/linux/randomize_kstack.h @@ -0,0 +1,98 @@ +/* SPDX-License-Identifier: GPL-2.0-only */ +#ifndef _LINUX_RANDOMIZE_KSTACK_H +#define _LINUX_RANDOMIZE_KSTACK_H + +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET +#include <linux/kernel.h> +#include <linux/jump_label.h> +#include <linux/percpu-defs.h> + +DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, + randomize_kstack_offset); +DECLARE_PER_CPU(u32, kstack_offset); + +/* + * Do not use this anywhere else in the kernel. This is used here because + * it provides an arch-agnostic way to grow the stack with correct + * alignment. Also, since this use is being explicitly masked to a max of + * 10 bits, stack-clash style attacks are unlikely. For more details see + * "VLAs" in Documentation/process/deprecated.rst + * + * The normal __builtin_alloca() is initialized with INIT_STACK_ALL (currently + * only with Clang and not GCC). Initializing the unused area on each syscall + * entry is expensive, and generating an implicit call to memset() may also be + * problematic (such as in noinstr functions). Therefore, if the compiler + * supports it (which it should if it initializes allocas), always use the + * "uninitialized" variant of the builtin. + */ +#if __has_builtin(__builtin_alloca_uninitialized) +#define __kstack_alloca __builtin_alloca_uninitialized +#else +#define __kstack_alloca __builtin_alloca +#endif + +/* + * Use, at most, 6 bits of entropy (on 64-bit; 8 on 32-bit). This cap is + * to keep the "VLA" from being unbounded (see above). Additionally clear + * the bottom 4 bits (on 64-bit systems, 2 for 32-bit), since stack + * alignment will always be at least word size. This makes the compiler + * code gen better when it is applying the actual per-arch alignment to + * the final offset. The resulting randomness is reasonable without overly + * constraining usable stack space. + */ +#ifdef CONFIG_64BIT +#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111110000) +#else +#define KSTACK_OFFSET_MAX(x) ((x) & 0b1111111100) +#endif + +/** + * add_random_kstack_offset - Increase stack utilization by previously + * chosen random offset + * + * This should be used in the syscall entry path when interrupts and + * preempt are disabled, and after user registers have been stored to + * the stack. For testing the resulting entropy, please see: + * tools/testing/selftests/lkdtm/stack-entropy.sh + */ +#define add_random_kstack_offset() do { \ + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ + &randomize_kstack_offset)) { \ + u32 offset = raw_cpu_read(kstack_offset); \ + u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ + /* Keep allocation even after "ptr" loses scope. */ \ + asm volatile("" :: "r"(ptr) : "memory"); \ + } \ +} while (0) + +/** + * choose_random_kstack_offset - Choose the random offset for the next + * add_random_kstack_offset() + * + * This should only be used during syscall exit when interrupts and + * preempt are disabled. This position in the syscall flow is done to + * frustrate attacks from userspace attempting to learn the next offset: + * - Maximize the timing uncertainty visible from userspace: if the + * offset is chosen at syscall entry, userspace has much more control + * over the timing between choosing offsets. "How long will we be in + * kernel mode?" tends to be more difficult to predict than "how long + * will we be in user mode?" + * - Reduce the lifetime of the new offset sitting in memory during + * kernel mode execution. Exposure of "thread-local" memory content + * (e.g. current, percpu, etc) tends to be easier than arbitrary + * location memory exposure. + */ +#define choose_random_kstack_offset(rand) do { \ + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ + &randomize_kstack_offset)) { \ + u32 offset = raw_cpu_read(kstack_offset); \ + offset = ror32(offset, 5) ^ (rand); \ + raw_cpu_write(kstack_offset, offset); \ + } \ +} while (0) +#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ +#define add_random_kstack_offset() do { } while (0) +#define choose_random_kstack_offset(rand) do { } while (0) +#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */ + +#endif |