From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755674AbbITXc4 (ORCPT ); Sun, 20 Sep 2015 19:32:56 -0400 Received: from mail.kernel.org ([198.145.29.136]:40051 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755543AbbITXcR (ORCPT ); Sun, 20 Sep 2015 19:32:17 -0400 From: Andy Lutomirski To: x86@kernel.org Cc: Sasha Levin , linux-kernel@vger.kernel.org, Andy Lutomirski , stable@vger.kernel.org Subject: [PATCH 2/2] x86/nmi/64: Fix a paravirt stack-clobbering bug in the NMI code Date: Sun, 20 Sep 2015 16:32:05 -0700 Message-Id: <974bc40edffdb5c2950a5c4977f821a446b76178.1442791737.git.luto@kernel.org> X-Mailer: git-send-email 2.4.3 In-Reply-To: References: In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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") Cc: stable@vger.kernel.org Reported-and-tested-by: Sasha Levin Signed-off-by: Andy Lutomirski --- arch/x86/entry/entry_64.S | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index f7492133a7bb..2c61e8070ba4 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1205,9 +1205,12 @@ 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(cpu_current_top_of_stack), %rsp -- 2.4.3