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/* 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
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