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Subject[patch 21/60] x86/entry/64: Remove the SYSENTER stack canary
From: Andy Lutomirski <luto@kernel.org>

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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/be3179c0a38c392fa44ebeb7dd89391ff5c010c3.1511497875.git.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(-)

--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -342,7 +342,6 @@ 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];

/*
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -48,8 +48,7 @@ bool in_sysenter_stack(unsigned long *st
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)
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -81,7 +81,6 @@
*/
.io_bitmap = { [0 ... IO_BITMAP_LONGS] = ~0 },
#endif
- .SYSENTER_stack_canary = STACK_END_MAGIC,
};
EXPORT_PER_CPU_SYMBOL(cpu_tss);

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -819,13 +819,6 @@ dotraplinkage void do_debug(struct pt_re
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);

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