In netfslib, a buffered writeback operation has a 'write queue' of folios
that are being written, held in a linear sequence of folio_queue structs.
The 'issuer' adds new folio_queues on the leading edge of the queue and
populates each one progressively; the 'collector' pops them off the
trailing edge and discards them and the folios they point to as they are
consumed.

The queue is required to always retain at least one folio_queue structure.
This allows the queue to be accessed without locking and with just a bit of
barriering.

When a new subrequest is prepared, its ->io_iter iterator is pointed at the
current end of the write queue and then the iterator is extended as more
data is added to the queue until the subrequest is committed.

Now, the problem is that the folio_queue at the leading edge of the write
queue when a subrequest is prepared might have been entirely consumed - but
not yet removed from the queue as it is the only remaining one and is
preventing the queue from collapsing.

So, what happens is that subreq->io_iter is pointed at the spent
folio_queue, then a new folio_queue is added, and, at that point, the
collector is at entirely at liberty to immediately delete the spent
folio_queue.

This leaves the subreq->io_iter pointing at a freed object.  If the system
is lucky, iterate_folioq() sees ->io_iter, sees the as-yet uncorrupted
freed object and advances to the next folio_queue in the queue.

In the case seen, however, the freed object gets recycled and put back onto
the queue at the tail and filled to the end.  This confuses
iterate_folioq() and it tries to step ->next, which may be NULL - resulting
in an oops.

Fix this by the following means:

 (1) When preparing a write subrequest, make sure there's a folio_queue
     struct with space in it at the leading edge of the queue.  A function
     to make space is split out of the function to append a folio so that
     it can be called for this purpose.

 (2) If the request struct iterator is pointing to a completely spent
     folio_queue when we make space, then advance the iterator to the newly
     allocated folio_queue.  The subrequest's iterator will then be set
     from this.

The oops could be triggered using the generic/346 xfstest with a filesystem
on9P over TCP with cache=loose.  The oops looked something like:

 BUG: kernel NULL pointer dereference, address: 0000000000000008
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 ...
 RIP: 0010:_copy_from_iter+0x2db/0x530
 ...
 Call Trace:
  <TASK>
 ...
  p9pdu_vwritef+0x3d8/0x5d0
  p9_client_prepare_req+0xa8/0x140
  p9_client_rpc+0x81/0x280
  p9_client_write+0xcf/0x1c0
  v9fs_issue_write+0x87/0xc0
  netfs_advance_write+0xa0/0xb0
  netfs_write_folio.isra.0+0x42d/0x500
  netfs_writepages+0x15a/0x1f0
  do_writepages+0xd1/0x220
  filemap_fdatawrite_wbc+0x5c/0x80
  v9fs_mmap_vm_close+0x7d/0xb0
  remove_vma+0x35/0x70
  vms_complete_munmap_vmas+0x11a/0x170
  do_vmi_align_munmap+0x17d/0x1c0
  do_vmi_munmap+0x13e/0x150
  __vm_munmap+0x92/0xd0
  __x64_sys_munmap+0x17/0x20
  do_syscall_64+0x80/0xe0
  entry_SYSCALL_64_after_hwframe+0x71/0x79

This also fixed a similar-looking issue with cifs and generic/074.

Fixes: cd0277ed ("netfs: Use new folio_queue data type and iterator instead of xarray iter")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202409180928.f20b5a08-oliver.sang@intel.com
Closes: https://lore.kernel.org/oe-lkp/202409131438.3f225fbf-oliver.sang@intel.comSigned-off-by: David Howells <dhowells@redhat.com>
Tested-by: kernel test robot <oliver.sang@intel.com>
cc: Eric Van Hensbergen <ericvh@kernel.org>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: v9fs@lists.linux.dev
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>