include/linux/randomize_kstack.h

/* SPDX-License-Identifier: GPL-2.0-only */
#ifndef _LINUX_RANDOMIZE_KSTACK_H
#define _LINUX_RANDOMIZE_KSTACK_H

#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
 */
void *__builtin_alloca(size_t size);
/*
 * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
 * "VLA" from being unbounded (see above). 10 bits leaves enough room for
 * per-arch offset masks to reduce entropy (by removing higher bits, since
 * high entropy may overly constrain usable stack space), and for
 * compiler/arch-specific stack alignment to remove the lower bits.
 */
#define KSTACK_OFFSET_MAX(x)	((x) & 0x3FF)

/*
 * These macros must be used during syscall entry when interrupts and
 * preempt are disabled, and after user registers have been stored to
 * the stack.
 */
#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 = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
		/* Keep allocation even after "ptr" loses scope. */	\
		asm volatile("" :: "r"(ptr) : "memory");		\
	}								\
} while (0)

#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 ^= (rand);					\
		raw_cpu_write(kstack_offset, offset);			\
	}								\
} while (0)

#endif