Re: [PATCH v3] loop: Fix NULL pointer dereference in lo_rw_aio()

[Ming Lei <tom.leiming@gmail.com>]

On Tue, Jun 02, 2026 at 05:51:26AM +0800, Hillf Danton wrote:
> On Mon, 1 Jun 2026 10:29:25 -0500 Ming Lei wrote:
> >On Thu, May 28, 2026 at 5:16 AM Qu Wenruo <wqu@suse.com> wrote:
> >> 在 2026/5/28 18:08, Christoph Hellwig 写道:
> >> > On Thu, May 28, 2026 at 03:11:05AM +0900, Damien Le Moal wrote:
> >> >> It sounds like the VFS unmount call needs to have something that waits for
> >> >> sync() to complete. Though, it really feels very strange that an FS can complete
> >> >
> >> > I don't think this is the VFS-controlled VFS file data writeback, which
> >> > we wait on, but some kind of fs controlled metadata.  And yes, it looks
> >> > like those file systems are buggy in that area.  We definitively had
> >> > such bugs in XFS before and fixed them.
> >> >
> >> > e.g. 9c7504aa72b6 ("xfs: track and serialize in-flight async buffers against
> >> > unmount")
> >> Considering the xfs fix is pretty old, it's before the fix hint thus no
> >> such mention in fstests.
> >>
> >> Do you happen to know which test case is for that fix?
> >> I'd like to adapt it for btrfs as a reproducer.
> >>
> >> This syzbot report doesn't provide a reproducer.
> >>
> >>
> >> Another thing is, if it's some btrfs bios on-the-fly after
> >> close_ctree(), the most common symptom should be NULL pointer
> >> dereference inside various btrfs endio functions.
> >> As all those end_bbio_*() functions are referring to either fs_info or
> >> inode/eb, thus if the fs is unmounted before the bio finished, they
> >> should all cause use-after-free.
> >>
> >> The only exception is discard, which is using blkdev_issue_discard()
> >> thus has no such reference to btrfs internal structure, but that's out
> >> of my understanding.
> >
> > syzbot log shows the null-ptr-deref  is on WRITE, instead of DISCARD.
> >
> > https://syzkaller.appspot.com/bug?extid=cd8a9a308e879a4e2c28
> >
> > Adding WARN_ON(!lo->lo_backing_file) in loop_queue_rq() might capture
> > this bio submission context if this req isn't issued via wq.
> >
> I suspect this makes $.02 sense given the check of Lo_bound upon queuing rq.

Can't lo->lo_state be updated after the check? It is totally lockless...


Thanks,
Ming