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Message-ID: <20230815060443.660263-1-chengming.zhou@linux.dev>
Date: Tue, 15 Aug 2023 14:04:42 +0800
From: chengming.zhou@...ux.dev
To: axboe@...nel.dk, kch@...dia.com, dhowells@...hat.com
Cc: damien.lemoal@...nsource.wdc.com, bvanassche@....org,
nj.shetty@...sung.com, kbusch@...nel.org,
zhouchengming@...edance.com, akinobu.mita@...il.com,
shinichiro.kawasaki@....com, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH] null_blk: fix poll request timeout handling
From: Chengming Zhou <zhouchengming@...edance.com>
When doing io_uring benchmark on /dev/nullb0, it's easy to crash the
kernel if poll requests timeout triggered, as reported by David. [1]
BUG: kernel NULL pointer dereference, address: 0000000000000008
Workqueue: kblockd blk_mq_timeout_work
RIP: 0010:null_timeout_rq+0x4e/0x91
Call Trace:
? __die_body+0x1a/0x5c
? page_fault_oops+0x6f/0x9c
? kernelmode_fixup_or_oops+0xc6/0xd6
? __bad_area_nosemaphore+0x44/0x1eb
? exc_page_fault+0xe2/0xf4
? asm_exc_page_fault+0x22/0x30
? null_timeout_rq+0x4e/0x91
blk_mq_handle_expired+0x31/0x4b
bt_iter+0x68/0x84
? bt_tags_iter+0x81/0x81
__sbitmap_for_each_set.constprop.0+0xb0/0xf2
? __blk_mq_complete_request_remote+0xf/0xf
bt_for_each+0x46/0x64
? __blk_mq_complete_request_remote+0xf/0xf
? percpu_ref_get_many+0xc/0x2a
blk_mq_queue_tag_busy_iter+0x14d/0x18e
blk_mq_timeout_work+0x95/0x127
process_one_work+0x185/0x263
worker_thread+0x1b5/0x227
? rescuer_thread+0x287/0x287
kthread+0xfa/0x102
? kthread_complete_and_exit+0x1b/0x1b
ret_from_fork+0x22/0x30
This is indeed a race problem between null_timeout_rq() and null_poll().
null_poll() null_timeout_rq()
spin_lock(&nq->poll_lock)
list_splice_init(&nq->poll_list, &list)
spin_unlock(&nq->poll_lock)
while (!list_empty(&list))
req = list_first_entry()
list_del_init()
...
blk_mq_add_to_batch()
// req->rq_next = NULL
spin_lock(&nq->poll_lock)
// rq->queuelist->next == NULL
list_del_init(&rq->queuelist)
spin_unlock(&nq->poll_lock)
What's worse is that we don't call blk_mq_complete_request_remote()
before blk_mq_add_to_batch(), so these completed requests have wrong
rq->state == MQ_RQ_IN_FLIGHT. We can easily check this using bpftrace:
```
bpftrace -e 'kretfunc:null_blk:null_poll {
$iob=(struct io_comp_batch *)args->iob;
@[$iob->req_list->state]=count();
}'
@[1]: 51708
```
Fix these problems by setting requests state to MQ_RQ_COMPLETE under
nq->poll_lock protection, in which null_timeout_rq() can safely detect
this race and early return.
[1] https://lore.kernel.org/all/3893581.1691785261@warthog.procyon.org.uk/
Fixes: 0a593fbbc245 ("null_blk: poll queue support")
Reported-by: David Howells <dhowells@...hat.com>
Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
drivers/block/null_blk/main.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index 864013019d6b..968090935eb2 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -1643,9 +1643,12 @@ static int null_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
struct nullb_queue *nq = hctx->driver_data;
LIST_HEAD(list);
int nr = 0;
+ struct request *rq;
spin_lock(&nq->poll_lock);
list_splice_init(&nq->poll_list, &list);
+ list_for_each_entry(rq, &list, queuelist)
+ blk_mq_set_request_complete(rq);
spin_unlock(&nq->poll_lock);
while (!list_empty(&list)) {
@@ -1671,16 +1674,21 @@ static enum blk_eh_timer_return null_timeout_rq(struct request *rq)
struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
struct nullb_cmd *cmd = blk_mq_rq_to_pdu(rq);
- pr_info("rq %p timed out\n", rq);
-
if (hctx->type == HCTX_TYPE_POLL) {
struct nullb_queue *nq = hctx->driver_data;
spin_lock(&nq->poll_lock);
+ /* The request may have completed meanwhile. */
+ if (blk_mq_request_completed(rq)) {
+ spin_unlock(&nq->poll_lock);
+ return BLK_EH_DONE;
+ }
list_del_init(&rq->queuelist);
spin_unlock(&nq->poll_lock);
}
+ pr_info("rq %p timed out\n", rq);
+
/*
* If the device is marked as blocking (i.e. memory backed or zoned
* device), the submission path may be blocked waiting for resources
--
2.41.0
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