[PATCH] nvme: use correct upper limit for tag in nvme_handle_cqe()

[Xianting Tian]

We met a crash issue when hot-insert a nvme device, blk_mq_tag_to_rq()
returned null(req=null), then crash happened in nvme_end_request():
	req = blk_mq_tag_to_rq();
        struct nvme_request *rq = nvme_req(req); //rq = req + 1
        rq->result = result;  <==crash here!!!

        [ 1124.256246] nvme nvme5: pci function 0000:e1:00.0
        [ 1124.256323] nvme 0000:e1:00.0: enabling device (0000 -> 0002)
        [ 1125.720859] nvme nvme5: 96/0/0 default/read/poll queues
        [ 1125.732483]  nvme5n1: p1 p2 p3
        [ 1125.788049] BUG: unable to handle kernel NULL pointer dereference at 0000000000000130
        [ 1125.788054] PGD 0 P4D 0
        [ 1125.788057] Oops: 0002 [#1] SMP NOPTI
        [ 1125.788059] CPU: 50 PID: 0 Comm: swapper/50 Kdump: loaded Tainted: G   ------- -t - 4.18.0-147.el8.x86_64 #1
        [ 1125.788065] RIP: 0010:nvme_irq+0xe8/0x240 [nvme]
        [ 1125.788068] RSP: 0018:ffff916b8ec83ed0 EFLAGS: 00010813
        [ 1125.788069] RAX: 0000000000000000 RBX: ffff918ae9211b00 RCX: 0000000000000000
        [ 1125.788070] RDX: 000000000000400b RSI: 0000000000000000 RDI: 0000000000000000
        [ 1125.788071] RBP: ffff918ae8870000 R08: 0000000000000004 R09: ffff918ae8870000
        [ 1125.788072] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
        [ 1125.788073] R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000001
        [ 1125.788075] FS:  0000000000000000(0000) GS:ffff916b8ec80000(0000) knlGS:0000000000000000
        [ 1125.788075] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
        [ 1125.788076] CR2: 0000000000000130 CR3: 0000001768f00000 CR4: 0000000000340ee0
        [ 1125.788077] Call Trace:
        [ 1125.788080]  <IRQ>
        [ 1125.788085]  __handle_irq_event_percpu+0x40/0x180
        [ 1125.788087]  handle_irq_event_percpu+0x30/0x80
        [ 1125.788089]  handle_irq_event+0x36/0x53
        [ 1125.788090]  handle_edge_irq+0x82/0x190
        [ 1125.788094]  handle_irq+0xbf/0x100
        [ 1125.788098]  do_IRQ+0x49/0xd0
        [ 1125.788100]  common_interrupt+0xf/0xf

The analysis of the possible cause of above crash as below.

According to our test, in nvme_pci_enable(), 'dev->q_depth' is set to 1024,
in nvme_create_io_queues()->nvme_alloc_queue(), 'nvmeq->q_depth' is set to
'dev->q_depth'. In nvme_dev_add(), 'dev->tagset.queue_depth' is set to 1023:
         dev->tagset.queue_depth = min_t(unsigned int, dev->q_depth, BLK_MQ_MAX_DEPTH) - 1;
					//why -1?? first involved by commit a4aea562
In nvme_alloc_queue(), 'nvmeq->q_depth' is set to dev->q_depth'.
So we got below values for mutiple depth:
dev->q_depth            = 1024
dev->tagset.queue_depth = 1023
nvmeq->q_depth          = 1024

In blk_mq_alloc_rqs(), it allocated 1023(dev->tagset.queue_depth) rqs for
one hw queue. In nvme_alloc_queue(), it allocated 1024(nvmeq->q_depth)
cqes for nvmeq->cqes[]. When process cqe in nvme_handle_cqe(), it get it by:
	struct nvme_completion *cqe = &nvmeq->cqes[idx];

We also added below prints in nvme_handle_cqe() and blk_mq_tag_to_rq():
	static inline void nvme_handle_cqe(struct nvme_queue *nvmeq, u16 idx)
	{
		volatile struct nvme_completion *cqe = &nvmeq->cqes[idx];
		struct request *req;

		//debug print
		dev_warn(nvmeq->dev->ctrl.device,
			"command_id %d completed on queue %d, nvmeq q_depth %d, nvme tagset q_depth %d\n",
		 	 cqe->command_id, le16_to_cpu(cqe->sq_id),
			 nvmeq->q_depth, nvmeq->dev->tagset.queue_depth);

		if (unlikely(cqe->command_id >= nvmeq->q_depth)) {
			dev_warn(nvmeq->dev->ctrl.device,
				"invalid id %d completed on queue %d\n",
				cqe->command_id, le16_to_cpu(cqe->sq_id));
			return;
		}

		... ...

		req = blk_mq_tag_to_rq(nvme_queue_tagset(nvmeq), cqe->command_id);
		... ...
	}

	struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
	{
		//debug print
		printk("tag, nr_tags:%d %d\n", tag, tags->nr_tags);
		if (tag < tags->nr_tags) {
			prefetch(tags->rqs[tag]);
			return tags->rqs[tag];
		}

		return NULL;
	}

According to the output, we know the max tag number(nr_tags) is 1023,  but
nvmeq->q_depth is 1024 and nvmeq->cqes[] has 1024 cqes. So if command_id(tag)
is 1023, the judgement "if (unlikely(cqe->command_id >= nvmeq->q_depth))"
in nvme_handle_cqe() is useless, we will get a null pointer, which is returned
by blk_mq_tag_to_rq():
	[   16.649973] nvme nvme0: command_id 968 completed on queue 13, nvmeq q_depth 1024, nvme tagset q_depth 1023
	[   16.649974] tag, nr_tags:968 1023

This patch is to make command_id match to its correct upper limit
'nvmeq->dev->tagset.queue_depth', not nvmeq->q_depth. So even if
we got 1023 of command_id, we can avoid a null pointer deference.

Signed-off-by: Xianting Tian <tian.xianting at h3c.com>
---
 drivers/nvme/host/pci.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
index 899d2f4d7..c681e26d0 100644
--- a/drivers/nvme/host/pci.c
+++ b/drivers/nvme/host/pci.c
@@ -940,7 +940,7 @@ static inline void nvme_handle_cqe(struct nvme_queue *nvmeq, u16 idx)
 	struct nvme_completion *cqe = &nvmeq->cqes[idx];
 	struct request *req;
 
-	if (unlikely(cqe->command_id >= nvmeq->q_depth)) {
+	if (unlikely(cqe->command_id >= nvmeq->dev->tagset.queue_depth)) {
 		dev_warn(nvmeq->dev->ctrl.device,
 			"invalid id %d completed on queue %d\n",
 			cqe->command_id, le16_to_cpu(cqe->sq_id));
-- 
2.17.1