Re: possible deadlock in get_user_pages_unlocked

[Al Viro]

On Fri, Feb 02, 2018 at 09:57:27AM +0100, Dmitry Vyukov wrote:

> syzbot tests for up to 5 minutes. However, if there is a race involved
> then you may need more time because the crash is probabilistic.
> But from what I see most of the time, if one can't reproduce it
> easily, it's usually due to some differences in setup that just don't
> allow the crash to happen at all.
> FWIW syzbot re-runs each reproducer on a freshly booted dedicated VM
> and what it provided is the kernel output it got during run of the
> provided program. So we have reasonably high assurance that this
> reproducer worked in at least one setup.

Could you guys check if the following fixes the reproducer?

diff --git a/mm/gup.c b/mm/gup.c
index 61015793f952..058a9a8e4e2e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -861,6 +861,9 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
 		BUG_ON(*locked != 1);
 	}
 
+	if (flags & FOLL_NOWAIT)
+		locked = NULL;
+
 	if (pages)
 		flags |= FOLL_GET;