Commit 83c133cf authored Sep 20, 2015 by Andy Lutomirski Committed by Thomas Gleixner Sep 22, 2015

x86/nmi/64: Fix a paravirt stack-clobbering bug in the NMI code



The NMI entry code that switches to the normal kernel stack needs to
be very careful not to clobber any extra stack slots on the NMI
stack.  The code is fine under the assumption that SWAPGS is just a
normal instruction, but that assumption isn't really true.  Use
SWAPGS_UNSAFE_STACK instead.

This is part of a fix for some random crashes that Sasha saw.

Fixes: 9b6e6a83 ("x86/nmi/64: Switch stacks on userspace NMI entry")
Reported-and-tested-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/974bc40edffdb5c2950a5c4977f821a446b76178.1442791737.git.luto@kernel.org


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent fc57a7c6

arch/x86/entry/entry_64.S

+4 −1

Original line number	Original line	Diff line number	Diff line
	@@ -1190,9 +1190,12 @@ ENTRY(nmi)
	* we don't want to enable interrupts, because then we'll end		* we don't want to enable interrupts, because then we'll end
	* up in an awkward situation in which IRQs are on but NMIs		* up in an awkward situation in which IRQs are on but NMIs
	* are off.		* are off.
			*
			* We also must not push anything to the stack before switching
			* stacks lest we corrupt the "NMI executing" variable.
	*/		*/

	SWAPGS		SWAPGS_UNSAFE_STACK
	cld		cld
	movq %rsp, %rdx		movq %rsp, %rdx
	movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp		movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp