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Commit 8d699663 authored by Yufen Yu's avatar Yufen Yu Committed by Jens Axboe
Browse files

block: fix null pointer dereference in blk_mq_rq_timed_out()

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 <>
Cc: Keith Busch <>
Cc: Bart Van Assche <>
Cc: # v4.18+
Reviewed-by: default avatarMing Lei <>
Reviewed-by: default avatarBob Liu <>
Signed-off-by: default avatarYufen Yu <>

 - 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 <>
parent 2af2783f
......@@ -214,6 +214,16 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
/* release the tag's ownership to the req cloned from */
spin_lock_irqsave(&fq->mq_flush_lock, flags);
if (!refcount_dec_and_test(&flush_rq->ref)) {
fq->rq_status = error;
spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
if (fq->rq_status != BLK_STS_OK)
error = fq->rq_status;
hctx = flush_rq->mq_hctx;
if (!q->elevator) {
blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq);
......@@ -918,7 +918,10 @@ static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
if (blk_mq_req_expired(rq, next))
blk_mq_rq_timed_out(rq, reserved);
if (refcount_dec_and_test(&rq->ref))
if (is_flush_rq(rq, hctx))
rq->end_io(rq, 0);
else if (refcount_dec_and_test(&rq->ref))
return true;
......@@ -19,6 +19,7 @@ struct blk_flush_queue {
unsigned int flush_queue_delayed:1;
unsigned int flush_pending_idx:1;
unsigned int flush_running_idx:1;
blk_status_t rq_status;
unsigned long flush_pending_since;
struct list_head flush_queue[2];
struct list_head flush_data_in_flight;
......@@ -47,6 +48,12 @@ static inline void __blk_get_queue(struct request_queue *q)
static inline bool
is_flush_rq(struct request *req, struct blk_mq_hw_ctx *hctx)
return hctx->fq->flush_rq == req;
struct blk_flush_queue *blk_alloc_flush_queue(struct request_queue *q,
int node, int cmd_size, gfp_t flags);
void blk_free_flush_queue(struct blk_flush_queue *q);
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