[3.16.y-ckt stable] Patch "x86/nmi/64: Fix a paravirt stack-clobbering bug in the NMI code" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt19.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 528d9753b30ef0e8fac9220c0e98bcb98ec11282 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto at kernel.org>
Date: Sun, 20 Sep 2015 16:32:05 -0700
Subject: 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 at oracle.com>
Signed-off-by: Andy Lutomirski <luto at kernel.org>
Link: http://lkml.kernel.org/r/974bc40edffdb5c2950a5c4977f821a446b76178.1442791737.git.luto@kernel.org
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
[ luis: backported to 3.16:
  - file rename: arch/x86/entry/entry_64.S -> arch/x86/kernel/entry_64.S
  - adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/x86/kernel/entry_64.S | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 47b8deb372f9..701db48562b1 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1489,9 +1489,12 @@ 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