[PATCH v5 0/6] handle unexpected message from server

[Yu Kuai]

This patch set tries to fix that client might oops if nbd server send
unexpected message to client, for example, our syzkaller report a uaf
in nbd_read_stat():

Call trace:
 dump_backtrace+0x0/0x310 arch/arm64/kernel/time.c:78
 show_stack+0x28/0x38 arch/arm64/kernel/traps.c:158
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x144/0x1b4 lib/dump_stack.c:118
 print_address_description+0x68/0x2d0 mm/kasan/report.c:253
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report+0x134/0x2f0 mm/kasan/report.c:409
 check_memory_region_inline mm/kasan/kasan.c:260 [inline]
 __asan_load4+0x88/0xb0 mm/kasan/kasan.c:699
 __read_once_size include/linux/compiler.h:193 [inline]
 blk_mq_rq_state block/blk-mq.h:106 [inline]
 blk_mq_request_started+0x24/0x40 block/blk-mq.c:644
 nbd_read_stat drivers/block/nbd.c:670 [inline]
 recv_work+0x1bc/0x890 drivers/block/nbd.c:749
 process_one_work+0x3ec/0x9e0 kernel/workqueue.c:2147
 worker_thread+0x80/0x9d0 kernel/workqueue.c:2302
 kthread+0x1d8/0x1e0 kernel/kthread.c:255
 ret_from_fork+0x10/0x18 arch/arm64/kernel/entry.S:1174

1) At first, a normal io is submitted and completed with scheduler:

internel_tag = blk_mq_get_tag -> get tag from sched_tags
 blk_mq_rq_ctx_init
  sched_tags->rq[internel_tag] = sched_tag->static_rq[internel_tag]
...
blk_mq_get_driver_tag
 __blk_mq_get_driver_tag -> get tag from tags
 tags->rq[tag] = sched_tag->static_rq[internel_tag]

So, both tags->rq[tag] and sched_tags->rq[internel_tag] are pointing
to the request: sched_tags->static_rq[internal_tag]. Even if the
io is finished.

2) nbd server send a reply with random tag directly:

recv_work
 nbd_read_stat
  blk_mq_tag_to_rq(tags, tag)
   rq = tags->rq[tag]

3) if the sched_tags->static_rq is freed:

blk_mq_sched_free_requests
 blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i)
  -> step 2) access rq before clearing rq mapping
  blk_mq_clear_rq_mapping(set, tags, hctx_idx);
  __free_pages() -> rq is freed here

4) Then, nbd continue to use the freed request in nbd_read_stat()

Changes in v5:
 - move patch 1 & 2 in v4 (patch 4 & 5 in v5) behind
 - add some comment in patch 5
Changes in v4:
 - change the name of the patchset, since uaf is not the only problem
 if server send unexpected reply message.
 - instead of adding new interface, use blk_mq_find_and_get_req().
 - add patch 5 to this series
Changes in v3:
 - v2 can't fix the problem thoroughly, add patch 3-4 to this series.
 - modify descriptions.
 - patch 5 is just a cleanup
Changes in v2:
 - as Bart suggested, add a new helper function for drivers to get
 request by tag.

Yu Kuai (6):
  nbd: don't handle response without a corresponding request message
  nbd: make sure request completion won't concurrent
  nbd: check sock index in nbd_read_stat()
  blk-mq: export two symbols to get request by tag
  nbd: convert to use blk_mq_find_and_get_req()
  nbd: don't start request if nbd_queue_rq() failed

 block/blk-mq-tag.c     |  5 +++--
 block/blk-mq.c         |  1 +
 drivers/block/nbd.c    | 51 ++++++++++++++++++++++++++++++++++++------
 include/linux/blk-mq.h |  3 +++
 4 files changed, 51 insertions(+), 9 deletions(-)

-- 
2.31.1