From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753788AbbGOVZV (ORCPT ); Wed, 15 Jul 2015 17:25:21 -0400 Received: from mail.kernel.org ([198.145.29.136]:59283 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753516AbbGOVZT (ORCPT ); Wed, 15 Jul 2015 17:25:19 -0400 From: Andy Lutomirski To: x86@kernel.org Cc: linux-kernel@vger.kernel.org, Linus Torvalds , Andy Lutomirski Subject: [PATCH] x86/entry: Fix _TIF_USER_RETURN_NOTIFY check in prepare_exit_to_usermode Date: Wed, 15 Jul 2015 14:25:16 -0700 Message-Id: <3f801104d24ee7a6bb1446408d9950777aa63277.1436995419.git.luto@kernel.org> X-Mailer: git-send-email 2.4.3 In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Linus noticed that the early return check was missing _TIF_USER_RETURN_NOTIFY. If the only work flag was _TIF_USER_RETURN_NOTIFY, we'd skip user return notifiers. Fix it. (This is the only missing bit.) This fixes double faults on a KVM host. It's the same issue as last time, except that this time it's very easy to trigger. Apparently no one uses -next as a KVM host. (I'm still not quite sure what it is that KVM does that blows up so badly if we miss a user return notifier. My best guess is that KVM lets KERNEL_GS_BASE (i.e. the user's gs base) be negative and fixes it up in a user return notifier. If we actually end up in user mode with a negative gs base, we blow up pretty badly.) Fixes: c5c46f59e4e7 ("x86/entry: Add new, comprehensible entry and exit handlers written in C") Signed-off-by: Andy Lutomirski --- arch/x86/entry/common.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index febc53086a69..a3e9c7fa15d9 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -264,7 +264,8 @@ __visible void prepare_exit_to_usermode(struct pt_regs *regs) READ_ONCE(pt_regs_to_thread_info(regs)->flags); if (!(cached_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | - _TIF_UPROBE | _TIF_NEED_RESCHED))) + _TIF_UPROBE | _TIF_NEED_RESCHED | + _TIF_USER_RETURN_NOTIFY))) break; /* We have work to do. */ -- 2.4.3