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

Josef Bacik
2021-10-15T16:08:38Z

On Thu, Sep 16, 2021 at 05:33:43PM +0800, Yu Kuai wrote:
> 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.
>

You can add

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

to the series. Sorry Jens, this one slipped through the cracks. Thanks,

Josef