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: c5c46f59e4 ("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>
d132803e6c