[patch 21/60] x86/entry/64: Remove the SYSENTER stack canary

[Thomas Gleixner]

From: Andy Lutomirski <luto@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Borislav Petkov <bp@xxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/be3179c0a38c392fa44ebeb7dd89391ff5c010c3.1511497875.git.luto@xxxxxxxxxx

---
 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);