On Fri, Feb 09, 2018 at 07:19:25PM -0800, Eric Biggers wrote: > Hi Al, > > On Sat, Feb 10, 2018 at 01:36:40AM +0000, Al Viro wrote: > > 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; > > > > Yes that fixes the reproducer for me. >
Just to follow up on this: it seems that Al's suggested fix didn't go anywhere, but someone else eventually ran into this bug (which was a real deadlock) and a slightly different fix was merged, commit 96312e61282ae. It fixes the reproducer for me too. Telling syzbot so that it can close the bug: #syz fix: mm/gup.c: teach get_user_pages_unlocked to handle FOLL_NOWAIT - Eric
