Patch "x86/nmi/64: Fix a paravirt stack-clobbering bug in the NMI code" has been added to the 4.1-stable tree

Patch "x86/nmi/64: Fix a paravirt stack-clobbering bug in the NMI code" has been added to the 4.1-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

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

to the 4.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-nmi-64-fix-a-paravirt-stack-clobbering-bug-in-the-nmi-code.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 83c133cf11fb0e68a51681447e372489f052d40e Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@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

From: Andy Lutomirski <luto@kernel.org>

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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/x86/kernel/entry_64.S |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1456,9 +1456,12 @@ 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


Patches currently in stable-queue which might be from luto@kernel.org are

queue-4.1/x86-paravirt-replace-the-paravirt-nop-with-a-bona-fide-empty-function.patch
queue-4.1/x86-nmi-64-fix-a-paravirt-stack-clobbering-bug-in-the-nmi-code.patch