Re: WARNING: lock held when returning to user space in fuse_lock_inode

[Alistair Strachan <astrachan@google.com>]

On Tue, Jul 17, 2018 at 5:46 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
> On Tue, Jul 17, 2018 at 1:36 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> > On Tue, Jul 17, 2018 at 1:14 PM, Miklos Szeredi <miklos@szeredi.hu> wrote:
> >> On Thu, Jul 12, 2018 at 5:49 PM, syzbot
> >> <syzbot+3f7b29af1baa9d0a55be@syzkaller.appspotmail.com> wrote:
> >>> Hello,
> >>>
> >>> syzbot found the following crash on:
> >>>
> >>> HEAD commit:    c25c74b7476e Merge tag 'trace-v4.18-rc3-2' of git://git.ke..
> >>> git tree:       upstream
> >>> console output: https://syzkaller.appspot.com/x/log.txt?x=177bcec2400000
> >>> kernel config:  https://syzkaller.appspot.com/x/.config?x=25856fac4e580aa7
> >>> dashboard link: https://syzkaller.appspot.com/bug?extid=3f7b29af1baa9d0a55be
> >>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> >>> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=13aa7678400000
> >>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17492678400000
> >>>
> >>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> >>> Reported-by: syzbot+3f7b29af1baa9d0a55be@syzkaller.appspotmail.com
> >>>
> >>> random: sshd: uninitialized urandom read (32 bytes read)
> >>> random: sshd: uninitialized urandom read (32 bytes read)
> >>> random: sshd: uninitialized urandom read (32 bytes read)
> >>>
> >>> ================================================
> >>> WARNING: lock held when returning to user space!
> >>> 4.18.0-rc4+ #143 Not tainted
> >>> ------------------------------------------------
> >>> syz-executor012/4539 is leaving the kernel with locks still held!
> >>> 1 lock held by syz-executor012/4539:
> >>>  #0: (____ptrval____) (&fi->mutex){+.+.}, at: fuse_lock_inode+0xaf/0xe0
> >>> fs/fuse/inode.c:363
> >>
> >> False positive.
> >>
> >> fi->mutex is definitely not held by the acquiring task when returning
> >> to userspace.  Maybe syzkaller is confused by the fact that there are
> >> several interdependent tasks involved with fuse:  the one calling into
> >> fuse by doing something (looking up ./file0/file0) and the one that
> >> reads the fuse device (returning with the LOOKUP request for "file0").
> >> The second one will return with that lock held, but it's not the one
> >> that acquired it, so there's no bug at all here.
> >
> > Hi Miklos,
> >
> > syzkaller is unrelated here. That's what kernel self-detects and
> > prints. So either way there is something to fix in kernel here: either
> > fuse or lockdep.
> >
> > +Alistair did some analysis offline, hope you don't mind if I repost
> > your description:
> > ===
> > Just from reading the code, I think I can see how this happens. Fuse
> > is wrapping its inode mutex with a check for "parallel_dirops", which
> > is set up in process_init_reply(). The FUSE_PARALLEL_DIROPS appears to
> > always be set, in fuse_send_init(), but its initial state is to be
> > disabled. So if the mutex gets taken, and it'll never be unlocked if
> > the initial command is flushed by fuse_readdir()'s use of
> > fuse_lock_inode().
> > ===
>
> Ah, indeed.  Fix attached.

Looks good to me.

Tested-by: Alistair Strachan <astrachan@google.com>

> Thanks,
> Miklos