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

@@ -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
 

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);
 

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);