syzbot has bisected this bug to:

commit e41d58185f1444368873d4d7422f7664a68be61d
Author: Dmitry Vyukov [off-list ref]
Date:   Wed Jul 12 21:34:35 2017 +0000

     fault-inject: support systematic fault injection

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=144b8772e00000
start commit:   8fe28cb5 Linux 4.20
git tree:       upstream
final crash:    https://syzkaller.appspot.com/x/report.txt?x=164b8772e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=124b8772e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7d581260bae0899a
dashboard link: https://syzkaller.appspot.com/bug?extid=6c5d567447bfa30f78e2
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=136130fd400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1607c563400000

Reported-by: syzbot+6c5d567447bfa30f78e2@syzkaller.appspotmail.com
Fixes: e41d58185f14 ("fault-inject: support systematic fault injection")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection