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/* SPDX-License-Identifier: GPL-2.0 */
/*
* GCC stack protector support.
*
* Stack protector works by putting predefined pattern at the start of
* the stack frame and verifying that it hasn't been overwritten when
* returning from the function. The pattern is called stack canary
* and unfortunately gcc historically required it to be at a fixed offset
* from the percpu segment base. On x86_64, the offset is 40 bytes.
*
* The same segment is shared by percpu area and stack canary. On
* x86_64, percpu symbols are zero based and %gs (64-bit) points to the
* base of percpu area. The first occupant of the percpu area is always
* fixed_percpu_data which contains stack_canary at the approproate
* offset. On x86_32, the stack canary is just a regular percpu
* variable.
*
* Putting percpu data in %fs on 32-bit is a minor optimization compared to
* using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
* to load 0 into %fs on exit to usermode, whereas with percpu data in
* %gs, we are likely to load a non-null %gs on return to user mode.
*
* Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
* support, we can remove some of this complexity.
*/
#ifndef _ASM_STACKPROTECTOR_H
#define _ASM_STACKPROTECTOR_H 1
#ifdef CONFIG_STACKPROTECTOR
#include <asm/tsc.h>
#include <asm/processor.h>
#include <asm/percpu.h>
#include <asm/desc.h>
#include <linux/random.h>
#include <linux/sched.h>
/*
* Initialize the stackprotector canary value.
*
* NOTE: this must only be called from functions that never return
* and it must always be inlined.
*
* In addition, it should be called from a compilation unit for which
* stack protector is disabled. Alternatively, the caller should not end
* with a function call which gets tail-call optimized as that would
* lead to checking a modified canary value.
*/
static __always_inline void boot_init_stack_canary(void)
{
u64 canary;
u64 tsc;
#ifdef CONFIG_X86_64
BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
#endif
/*
* We both use the random pool and the current TSC as a source
* of randomness. The TSC only matters for very early init,
* there it already has some randomness on most systems. Later
* on during the bootup the random pool has true entropy too.
*/
get_random_bytes(&canary, sizeof(canary));
tsc = rdtsc();
canary += tsc + (tsc << 32UL);
canary &= CANARY_MASK;
current->stack_canary = canary;
#ifdef CONFIG_X86_64
this_cpu_write(fixed_percpu_data.stack_canary, canary);
#else
this_cpu_write(__stack_chk_guard, canary);
#endif
}
static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
{
#ifdef CONFIG_X86_64
per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
#else
per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
#endif
}
#else /* STACKPROTECTOR */
/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
{ }
#endif /* STACKPROTECTOR */
#endif /* _ASM_STACKPROTECTOR_H */
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