[PATCH 20/21] x86/entry/64: Remove the SYSENTER stack canary

[Ingo Molnar <mingo@kernel.org>]

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: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Link: https://lkml.kernel.org/r/be3179c0a38c392fa44ebeb7dd89391ff5c010c3.1511497875.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@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 2f6d5634da55..806011202733 100644
--- 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];
 
 	/*
diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 47ea8e0daf81..35876e94286d 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 2cf7e2aa1730..298be43d63de 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 450688c75b3b..de4e3c80cf34 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -819,13 +819,6 @@ 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);
-- 
2.14.1