[patch v8 0/7] handle unexpected message from server

Yu Kuai
2021-09-16T09:24:05Z

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 v8:
- add patch 5 to this series.
- modify some words.
Changes in v7:
- instead of exposing blk_queue_exit(), using percpu_ref_put()
directly.
- drop the ref right after nbd_handle_reply().
Changes in v6:
- don't set cmd->status to error if request is completed before
nbd_clear_req().
- get 'q_usage_counter' to prevent accessing freed request through
blk_mq_tag_to_rq(), instead of using blk_mq_find_and_get_req().
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 (7):
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()
nbd: don't start request if nbd_queue_rq() failed
nbd: clean up return value checking of sock_xmit()
nbd: partition nbd_read_stat() into nbd_read_reply() and
nbd_handle_reply()
nbd: fix uaf in nbd_handle_reply()

drivers/block/nbd.c | 135 +++++++++++++++++++++++++++++++-------------
1 file changed, 96 insertions(+), 39 deletions(-)

--
2.31.1