x86/nmi: Test saved %cs in NMI to determine nested NMI case

Currently, the NMI handler tests if it is nested by checking the
special variable saved on the stack (set during NMI handling)
and whether the saved stack is the NMI stack as well (to prevent
the race when the variable is set to zero).

But userspace may set their %rsp to any value as long as they do
not derefence it, and it may make it point to the NMI stack,
which will prevent NMIs from triggering while the userspace app
is running. (I tested this, and it is indeed the case)

Add another check to determine nested NMIs by looking at the
saved %cs (code segment register) and making sure that it is the
kernel code segment.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@kernel.org>
Link: http://lkml.kernel.org/r/1329687817.1561.27.camel@acer.local.homeSigned-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/entry_64.S

@@ -1531,6 +1531,13 @@ ENTRY(nmi)
 	/* Use %rdx as out temp variable throughout */
 	pushq_cfi %rdx
 
+	/*
+	 * If %cs was not the kernel segment, then the NMI triggered in user
+	 * space, which means it is definitely not nested.
+	 */
+	cmp $__KERNEL_CS, 16(%rsp)
+	jne first_nmi
+
 	/*
 	 * Check the special variable on the stack to see if NMIs are
 	 * executing.