• Yufen Yu's avatar
    block: fix null pointer dereference in blk_mq_rq_timed_out() · 8d699663
    Yufen Yu authored
    We got a null pointer deference BUG_ON in blk_mq_rq_timed_out()
    as following:
    [  108.825472] BUG: kernel NULL pointer dereference, address: 0000000000000040
    [  108.827059] PGD 0 P4D 0
    [  108.827313] Oops: 0000 [#1] SMP PTI
    [  108.827657] CPU: 6 PID: 198 Comm: kworker/6:1H Not tainted 5.3.0-rc8+ #431
    [  108.829503] Workqueue: kblockd blk_mq_timeout_work
    [  108.829913] RIP: 0010:blk_mq_check_expired+0x258/0x330
    [  108.838191] Call Trace:
    [  108.838406]  bt_iter+0x74/0x80
    [  108.838665]  blk_mq_queue_tag_busy_iter+0x204/0x450
    [  108.839074]  ? __switch_to_asm+0x34/0x70
    [  108.839405]  ? blk_mq_stop_hw_queue+0x40/0x40
    [  108.839823]  ? blk_mq_stop_hw_queue+0x40/0x40
    [  108.840273]  ? syscall_return_via_sysret+0xf/0x7f
    [  108.840732]  blk_mq_timeout_work+0x74/0x200
    [  108.841151]  process_one_work+0x297/0x680
    [  108.841550]  worker_thread+0x29c/0x6f0
    [  108.841926]  ? rescuer_thread+0x580/0x580
    [  108.842344]  kthread+0x16a/0x1a0
    [  108.842666]  ? kthread_flush_work+0x170/0x170
    [  108.843100]  ret_from_fork+0x35/0x40
    The bug is caused by the race between timeout handle and completion for
    flush request.
    When timeout handle function blk_mq_rq_timed_out() try to read
    'req->q->mq_ops', the 'req' have completed and reinitiated by next
    flush request, which would call blk_rq_init() to clear 'req' as 0.
    After commit 12f5b931
     ("blk-mq: Remove generation seqeunce"),
    normal requests lifetime are protected by refcount. Until 'rq->ref'
    drop to zero, the request can really be free. Thus, these requests
    cannot been reused before timeout handle finish.
    However, flush request has defined .end_io and rq->end_io() is still
    called even if 'rq->ref' doesn't drop to zero. After that, the 'flush_rq'
    can be reused by the next flush request handle, resulting in null
    pointer deference BUG ON.
    We fix this problem by covering flush request with 'rq->ref'.
    If the refcount is not zero, flush_end_io() return and wait the
    last holder recall it. To record the request status, we add a new
    entry 'rq_status', which will be used in flush_end_io().
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: Keith Busch <keith.busch@intel.com>
    Cc: Bart Van Assche <bvanassche@acm.org>
    Cc: stable@vger.kernel.org # v4.18+
    Reviewed-by: default avatarMing Lei <ming.lei@redhat.com>
    Reviewed-by: default avatarBob Liu <bob.liu@oracle.com>
    Signed-off-by: default avatarYufen Yu <yuyufen@huawei.com>
     - move rq_status from struct request to struct blk_flush_queue
     - remove unnecessary '{}' pair.
     - let spinlock to protect 'fq->rq_status'
     - move rq_status after flush_running_idx member of struct blk_flush_queue
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>