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Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576777879808182838485/* 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>#include <linux/prandom.h>
DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, randomize_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
DECLARE_PER_CPU(struct rnd_state, kstack_rnd_state);
static __always_inline u32 get_kstack_offset(void){ struct rnd_state *state; u32 rnd;
state = &get_cpu_var(kstack_rnd_state); rnd = prandom_u32_state(state); put_cpu_var(kstack_rnd_state);
return rnd;}
/** * add_random_kstack_offset - Increase stack utilization by a random offset. * * This should be used in the syscall entry path after user registers have been * stored to the stack. Preemption may be enabled. 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 = get_kstack_offset(); \ u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset)); \ /* Keep allocation even after "ptr" loses scope. */ \ asm volatile("" :: "r"(ptr) : "memory"); \ } \} while (0)
#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */#define add_random_kstack_offset() do { } while (0)#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
#endif