[PATCH 2/2] nbd: hold tags->lock to prevent access freed request through blk_mq_tag_to_rq()

[Yu Kuai]

Our test reported a uaf problem:

Read of size 4 at addr ffff80036b790b54 by task kworker/u9:1/31105

Workqueue: knbd0-recv recv_work
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

This is because tags->static_rq can be freed without clearing tags->rq,
Ming Lei had fixed the problem while itering tags, howerver, the problem
still exist in blk_mq_tag_to_rq().

Thus fix the problem by holding tags->lock, so that tags->rq can be
cleared before tags->static_rq is freed.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/block/nbd.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index c38317979f74..c7ca16f0adbd 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -712,12 +712,22 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
 	memcpy(&handle, reply.handle, sizeof(handle));
 	tag = nbd_handle_to_tag(handle);
 	hwq = blk_mq_unique_tag_to_hwq(tag);
-	if (hwq < nbd->tag_set.nr_hw_queues)
-		req = blk_mq_tag_to_rq(nbd->tag_set.tags[hwq],
-				       blk_mq_unique_tag_to_tag(tag));
-	if (!req || !blk_mq_request_started(req)) {
-		dev_err(disk_to_dev(nbd->disk), "Unexpected reply (%d) %p\n",
-			tag, req);
+	if (hwq < nbd->tag_set.nr_hw_queues) {
+		unsigned long flags;
+		struct blk_mq_tags *tags = nbd->tag_set.tags[hwq];
+
+		blk_mq_tags_lock(tags, &flags);
+		req = blk_mq_tag_to_rq(tags, blk_mq_unique_tag_to_tag(tag));
+		if (!blk_mq_request_started(req)) {
+			dev_err(disk_to_dev(nbd->disk), "Request not started (%d) %p\n",
+				tag, req);
+			req = NULL;
+		}
+		blk_mq_tags_unlock(tags, &flags);
+	}
+
+	if (!req) {
+		dev_err(disk_to_dev(nbd->disk), "Unexpected reply (%d)\n", tag);
 		return ERR_PTR(-ENOENT);
 	}
 	trace_nbd_header_received(req, handle);
-- 
2.31.1