[PATCH] x86/entry: Fix _TIF_USER_RETURN_NOTIFY check in prepare_exit_to_usermode

[Andy Lutomirski <luto@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 <luto@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 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