[PATCH v3 18/19] x86/entry/64: Remove the SYSENTER stack canary

[Andy Lutomirski]

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 <l...@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