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622754e84b
The RNG always mixes in the Linux version extremely early in boot. It also always includes a cycle counter, not only during early boot, but each and every time it is invoked prior to being fully initialized. Together, this means that the use of additional xors inside of the various stackprotector.h files is superfluous and over-complicated. Instead, we can get exactly the same thing, but better, by just calling `get_random_canary()`. Acked-by: Guo Ren <guoren@kernel.org> # for csky Acked-by: Catalin Marinas <catalin.marinas@arm.com> # for arm64 Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
83 lines
2.7 KiB
C
83 lines
2.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* GCC stack protector support.
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*
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* Stack protector works by putting predefined pattern at the start of
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* the stack frame and verifying that it hasn't been overwritten when
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* returning from the function. The pattern is called stack canary
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* and unfortunately gcc historically required it to be at a fixed offset
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* from the percpu segment base. On x86_64, the offset is 40 bytes.
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*
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* The same segment is shared by percpu area and stack canary. On
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* x86_64, percpu symbols are zero based and %gs (64-bit) points to the
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* base of percpu area. The first occupant of the percpu area is always
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* fixed_percpu_data which contains stack_canary at the appropriate
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* offset. On x86_32, the stack canary is just a regular percpu
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* variable.
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*
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* Putting percpu data in %fs on 32-bit is a minor optimization compared to
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* using %gs. Since 32-bit userspace normally has %fs == 0, we are likely
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* to load 0 into %fs on exit to usermode, whereas with percpu data in
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* %gs, we are likely to load a non-null %gs on return to user mode.
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*
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* Once we are willing to require GCC 8.1 or better for 64-bit stackprotector
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* support, we can remove some of this complexity.
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*/
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#ifndef _ASM_STACKPROTECTOR_H
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#define _ASM_STACKPROTECTOR_H 1
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#ifdef CONFIG_STACKPROTECTOR
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#include <asm/tsc.h>
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#include <asm/processor.h>
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#include <asm/percpu.h>
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#include <asm/desc.h>
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#include <linux/sched.h>
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/*
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* Initialize the stackprotector canary value.
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*
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* NOTE: this must only be called from functions that never return
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* and it must always be inlined.
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*
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* In addition, it should be called from a compilation unit for which
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* stack protector is disabled. Alternatively, the caller should not end
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* with a function call which gets tail-call optimized as that would
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* lead to checking a modified canary value.
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*/
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static __always_inline void boot_init_stack_canary(void)
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{
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unsigned long canary = get_random_canary();
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#ifdef CONFIG_X86_64
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BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
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#endif
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current->stack_canary = canary;
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#ifdef CONFIG_X86_64
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this_cpu_write(fixed_percpu_data.stack_canary, canary);
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#else
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this_cpu_write(__stack_chk_guard, canary);
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#endif
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}
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static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
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{
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#ifdef CONFIG_X86_64
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per_cpu(fixed_percpu_data.stack_canary, cpu) = idle->stack_canary;
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#else
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per_cpu(__stack_chk_guard, cpu) = idle->stack_canary;
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#endif
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}
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#else /* STACKPROTECTOR */
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/* dummy boot_init_stack_canary() is defined in linux/stackprotector.h */
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static inline void cpu_init_stack_canary(int cpu, struct task_struct *idle)
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{ }
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#endif /* STACKPROTECTOR */
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#endif /* _ASM_STACKPROTECTOR_H */
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