[tip:x86/asm] x86/entry: Fix _TIF_USER_RETURN_NOTIFY check in prepare_exit_to_usermode

tip-bot for Andy Lutomirski
2015-07-18T03:25:55Z

Commit-ID: d132803e6c611d50c19baedc8ae520203a2baca7
Gitweb: http://git.kernel.org/tip/d132803e6c611d50c19baedc8ae520203a2baca7
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Wed, 15 Jul 2015 14:25:16 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 17 Jul 2015 16:08:22 +0200

x86/entry: Fix _TIF_USER_RETURN_NOTIFY check in prepare_exit_to_usermode

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. )

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: c5c46f59e4e7 ("x86/entry: Add new, comprehensible entry and exit handlers written in C")
Link: http://lkml.kernel.org/r/3f801104d24ee7a6bb1446408d9950777aa63277.1436995419.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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 febc530..a3e9c7f 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. */