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

commit 83c133cf11fb0e68a51681447e372489f052d40e upstream.

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: 9b6e6a8334d5 ("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>
Link: http://lkml.kernel.org/r/974bc40edffdb5c2950a5c4977f821a446b76178.1442791737.git.luto@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 7ed99df028ca60c3cd44821c0615a47bb278ff0d..2bafcb32941e7a1a614f66a0ee0af2c7734800bc 100644 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1727,9 +1727,12 @@ ENTRY(nmi)
         * we don't want to enable interrupts, because then we'll end
         * up in an awkward situation in which IRQs are on but NMIs
         * 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
        movq    %rsp, %rdx
        movq    PER_CPU_VAR(kernel_stack), %rsp