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3fb0fdb3bb
On 32-bit kernels, the stackprotector canary is quite nasty -- it is stored at %gs:(20), which is nasty because 32-bit kernels use %fs for percpu storage. It's even nastier because it means that whether %gs contains userspace state or kernel state while running kernel code depends on whether stackprotector is enabled (this is CONFIG_X86_32_LAZY_GS), and this setting radically changes the way that segment selectors work. Supporting both variants is a maintenance and testing mess. Merely rearranging so that percpu and the stack canary share the same segment would be messy as the 32-bit percpu address layout isn't currently compatible with putting a variable at a fixed offset. Fortunately, GCC 8.1 added options that allow the stack canary to be accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary percpu variable. This lets us get rid of all of the code to manage the stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess. (That name is special. We could use any symbol we want for the %fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any name other than __stack_chk_guard.) Forcibly disable stackprotector on older compilers that don't support the new options and turn the stack canary into a percpu variable. The "lazy GS" approach is now used for all 32-bit configurations. Also makes load_gs_index() work on 32-bit kernels. On 64-bit kernels, it loads the GS selector and updates the user GSBASE accordingly. (This is unchanged.) On 32-bit kernels, it loads the GS selector and updates GSBASE, which is now always the user base. This means that the overall effect is the same on 32-bit and 64-bit, which avoids some ifdeffery. [ bp: Massage commit message. ] Signed-off-by: Andy Lutomirski <luto@kernel.org> Signed-off-by: Borislav Petkov <bp@suse.de> Link: https://lkml.kernel.org/r/c0ff7dba14041c7e5d1cae5d4df052f03759bef3.1613243844.git.luto@kernel.org
95 lines
3 KiB
C
95 lines
3 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 approproate
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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/random.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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u64 canary;
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u64 tsc;
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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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/*
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* We both use the random pool and the current TSC as a source
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* of randomness. The TSC only matters for very early init,
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* there it already has some randomness on most systems. Later
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* on during the bootup the random pool has true entropy too.
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*/
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get_random_bytes(&canary, sizeof(canary));
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tsc = rdtsc();
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canary += tsc + (tsc << 32UL);
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canary &= CANARY_MASK;
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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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