netfs: Report on NULL folioq in netfs_writeback_unlock_folios()

netfs: Report on NULL folioq in netfs_writeback_unlock_folios()

It seems that it's possible to get to netfs_writeback_unlock_folios() with
an empty rolling buffer during buffered writes.  This should not be
possible as the rolling buffer is initialised as the write request is set
up and thereafter maintains at least one folio_queue struct therein until
it gets destroyed.  This allows lockless addition and removal of
folio_queue structs in the buffer because, unlike with a ring buffer, the
producer and consumer each only need to look at and alter one pointer into
the buffer.

Now, the rolling buffer is only used for buffered I/O operations as
netfs_collect_write_results() should only call
netfs_writeback_unlock_folios() if the request is of origin type
NETFS_WRITEBACK, NETFS_WRITETHROUGH or NETFS_PGPRIV2_COPY_TO_CACHE.

So it would seem that one of the following occurred: (1) I/O started before
the request was fully initialised, (2) the origin got switched mid-flow or
(3) the request has already been freed and this is a UAF error.  I think
the last is the most likely.

Make netfs_writeback_unlock_folios() report information about the request
and subrequests if folioq is seen to be NULL to try and help debug this,
throw a warning and return.

Note that this does not try to fix the problem.

Reported-by: syzbot+af5c06208fa71bf31b16@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=af5c06208fa71bf31b16
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/ZxshMEW4U7MTgQYa@gmail.com/
Link: https://lore.kernel.org/r/20241216204124.3752367-33-dhowells@redhat.com
cc: Chang Yu <marcus.yu.56@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>