[tip:x86/asm] x86/entry/32: Add and check a stack canary for the SYSENTER stack

[tip-bot for Andy Lutomirski <tipbot@zytor.com>]

Commit-ID:  2a41aa4feb25af3ead60b740c43df80c576efea2
Gitweb:     http://git.kernel.org/tip/2a41aa4feb25af3ead60b740c43df80c576efea2
Author:     Andy Lutomirski <luto@kernel.org>
AuthorDate: Wed, 9 Mar 2016 19:00:33 -0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 10 Mar 2016 09:48:14 +0100

x86/entry/32: Add and check a stack canary for the SYSENTER stack

The first instruction of the SYSENTER entry runs on its own tiny
stack.  That stack can be used if a #DB or NMI is delivered before
the SYSENTER prologue switches to a real stack.

We have code in place to prevent us from overflowing the tiny stack.
For added paranoia, add a canary to the stack and check it in
do_debug() -- that way, if something goes wrong with the #DB logic,
we'll eventually notice.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/6ff9a806f39098b166dc2c41c1db744df5272f29.1457578375.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/processor.h | 3 ++-
 arch/x86/kernel/process.c        | 3 +++
 arch/x86/kernel/traps.c          | 8 ++++++++
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 7cd01b7..50a6dc8 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -299,8 +299,9 @@ struct tss_struct {
 
 #ifdef CONFIG_X86_32
 	/*
-	 * Space for the temporary SYSENTER stack:
+	 * Space for the temporary SYSENTER stack.
 	 */
+	unsigned long		SYSENTER_stack_canary;
 	unsigned long		SYSENTER_stack[64];
 #endif
 
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 9f7c21c..ee9a979 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -57,6 +57,9 @@ __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
 	  */
 	.io_bitmap		= { [0 ... IO_BITMAP_LONGS] = ~0 },
 #endif
+#ifdef CONFIG_X86_32
+	.SYSENTER_stack_canary	= STACK_END_MAGIC,
+#endif
 };
 EXPORT_PER_CPU_SYMBOL(cpu_tss);
 
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index b0ddb819..49e2e77 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -713,6 +713,14 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	debug_stack_usage_dec();
 
 exit:
+#if defined(CONFIG_X86_32)
+	/*
+	 * 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");
+#endif
 	ist_exit(regs);
 }
 NOKPROBE_SYMBOL(do_debug);