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    Subject[PATCH v3 18/19] x86/entry/64: Remove the SYSENTER stack canary
    Date
    Now that the SYSENTER stack has a guard page, there's no need for a
    canary to detect overflow after the fact.

    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    ---
    arch/x86/include/asm/processor.h | 1 -
    arch/x86/kernel/dumpstack.c | 3 +--
    arch/x86/kernel/process.c | 1 -
    arch/x86/kernel/traps.c | 7 -------
    4 files changed, 1 insertion(+), 11 deletions(-)

    diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
    index 3a09e5571a92..7743aedb82ea 100644
    --- a/arch/x86/include/asm/processor.h
    +++ b/arch/x86/include/asm/processor.h
    @@ -337,7 +337,6 @@ struct tss_struct {
    * Space for the temporary SYSENTER stack, used for SYSENTER
    * and the entry trampoline as well.
    */
    - unsigned long SYSENTER_stack_canary;
    unsigned long SYSENTER_stack[64];

    /*
    diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
    index bb61919c9335..9ce5fcf7d14d 100644
    --- a/arch/x86/kernel/dumpstack.c
    +++ b/arch/x86/kernel/dumpstack.c
    @@ -48,8 +48,7 @@ bool in_sysenter_stack(unsigned long *stack, struct stack_info *info)
    int cpu = smp_processor_id();
    struct tss_struct *tss = &get_cpu_entry_area(cpu)->tss;

    - /* Treat the canary as part of the stack for unwinding purposes. */
    - void *begin = &tss->SYSENTER_stack_canary;
    + void *begin = &tss->SYSENTER_stack;
    void *end = (void *)&tss->SYSENTER_stack + sizeof(tss->SYSENTER_stack);

    if ((void *)stack < begin || (void *)stack >= end)
    diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
    index 86e83762e3b3..6a04287f222b 100644
    --- a/arch/x86/kernel/process.c
    +++ b/arch/x86/kernel/process.c
    @@ -81,7 +81,6 @@ __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
    */
    .io_bitmap = { [0 ... IO_BITMAP_LONGS] = ~0 },
    #endif
    - .SYSENTER_stack_canary = STACK_END_MAGIC,
    };
    EXPORT_PER_CPU_SYMBOL(cpu_tss);

    diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
    index cbc4272bb9dd..19475dbff068 100644
    --- a/arch/x86/kernel/traps.c
    +++ b/arch/x86/kernel/traps.c
    @@ -801,13 +801,6 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
    debug_stack_usage_dec();

    exit:
    - /*
    - * This is the most likely code path that involves non-trivial use
    - * of the SYSENTER stack. Check that we haven't overrun it.
    - */
    - WARN(this_cpu_read(cpu_tss.SYSENTER_stack_canary) != STACK_END_MAGIC,
    - "Overran or corrupted SYSENTER stack\n");
    -
    ist_exit(regs);
    }
    NOKPROBE_SYMBOL(do_debug);
    --
    2.13.6
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