[PATCH v3 0/5] fix request uaf in nbd_read_stat()

[Yu Kuai]

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.


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)
  blk_mq_clear_rq_mapping(set, tags, hctx_idx);

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

This patchset try to fix the problem by following apporch:

 - add a new interface blk_mq_get_rq_by_tag() to replace blk_mq_tag_to_rq(),
 it will make sure the return request is started and won't be freed.
 - nbd client won't handle the reply if it didn't send the corresponding
 request message.
 - nbd won't complete a request multiple times

Yu Kuai (5):
  blk-mq: add a new interface to get request by tag
  nbd: convert to use blk_mq_get_rq_by_tag()
  nbd: don't handle response without a corresponding request message
  nbd: make sure request completion won't concurrent
  nbd: don't start request if nbd_queue_rq() failed

 block/blk-mq-tag.c     | 37 +++++++++++++++++++++++++++++++++++++
 block/blk-mq.c         |  1 +
 block/blk-mq.h         |  1 -
 drivers/block/nbd.c    | 39 +++++++++++++++++++++++++++++----------
 include/linux/blk-mq.h |  4 ++++
 5 files changed, 71 insertions(+), 11 deletions(-)

-- 
2.31.1