[PATCH] nbd: fix possible page fault for nbd disk

[xiubli@redhat.com]

From: Xiubo Li <xiubli@redhat.com>

When the NBD_CFLAG_DESTROY_ON_DISCONNECT flag is set and at the same
time when the socket is closed due to the server daemon is restarted,
there will be crashing randomly, like:

<3>[  110.151949] block nbd1: Receive control failed (result -32)
<1>[  110.152024] BUG: unable to handle page fault for address: 0000058000000840
<1>[  110.152063] #PF: supervisor read access in kernel mode
<1>[  110.152083] #PF: error_code(0x0000) - not-present page
<6>[  110.152094] PGD 0 P4D 0
<4>[  110.152106] Oops: 0000 [#1] SMP PTI
<4>[  110.152120] CPU: 0 PID: 6698 Comm: kworker/u5:1 Kdump: loaded Not tainted 5.3.0-rc4+ #2
<4>[  110.152136] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
<4>[  110.152166] Workqueue: knbd-recv recv_work [nbd]
<4>[  110.152187] RIP: 0010:__dev_printk+0xd/0x67
<4>[  110.152206] Code: 10 e8 c5 fd ff ff 48 8b 4c 24 18 65 48 33 0c 25 28 00 [...]
<4>[  110.152244] RSP: 0018:ffffa41581f13d18 EFLAGS: 00010206
<4>[  110.152256] RAX: ffffa41581f13d30 RBX: ffff96dd7374e900 RCX: 0000000000000000
<4>[  110.152271] RDX: ffffa41581f13d20 RSI: 00000580000007f0 RDI: ffffffff970ec24f
<4>[  110.152285] RBP: ffffa41581f13d80 R08: ffff96dd7fc17908 R09: 0000000000002e56
<4>[  110.152299] R10: ffffffff970ec24f R11: 0000000000000003 R12: ffff96dd7374e900
<4>[  110.152313] R13: 0000000000000000 R14: ffff96dd7374e9d8 R15: ffff96dd6e3b02c8
<4>[  110.152329] FS:  0000000000000000(0000) GS:ffff96dd7fc00000(0000) knlGS:0000000000000000
<4>[  110.152362] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4>[  110.152383] CR2: 0000058000000840 CR3: 0000000067cc6002 CR4: 00000000001606f0
<4>[  110.152401] Call Trace:
<4>[  110.152422]  _dev_err+0x6c/0x83
<4>[  110.152435]  nbd_read_stat.cold+0xda/0x578 [nbd]
<4>[  110.152448]  ? __switch_to_asm+0x34/0x70
<4>[  110.152468]  ? __switch_to_asm+0x40/0x70
<4>[  110.152478]  ? __switch_to_asm+0x34/0x70
<4>[  110.152491]  ? __switch_to_asm+0x40/0x70
<4>[  110.152501]  ? __switch_to_asm+0x34/0x70
<4>[  110.152511]  ? __switch_to_asm+0x40/0x70
<4>[  110.152522]  ? __switch_to_asm+0x34/0x70
<4>[  110.152533]  recv_work+0x35/0x9e [nbd]
<4>[  110.152547]  process_one_work+0x19d/0x340
<4>[  110.152558]  worker_thread+0x50/0x3b0
<4>[  110.152568]  kthread+0xfb/0x130
<4>[  110.152577]  ? process_one_work+0x340/0x340
<4>[  110.152609]  ? kthread_park+0x80/0x80
<4>[  110.152637]  ret_from_fork+0x35/0x40

This is very easy to reproduce by running the nbd-runner.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
---
 drivers/block/nbd.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index e21d2ded732b..bf5e4227c54d 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -26,6 +26,7 @@
 #include <linux/ioctl.h>
 #include <linux/mutex.h>
 #include <linux/compiler.h>
+#include <linux/completion.h>
 #include <linux/err.h>
 #include <linux/kernel.h>
 #include <linux/slab.h>
@@ -112,6 +113,8 @@ struct nbd_device {
 	struct list_head list;
 	struct task_struct *task_recv;
 	struct task_struct *task_setup;
+
+	struct completion complete;
 };
 
 #define NBD_CMD_REQUEUED	1
@@ -231,6 +234,13 @@ static void nbd_put(struct nbd_device *nbd)
 					&nbd_index_mutex)) {
 		idr_remove(&nbd_index_idr, nbd->index);
 		mutex_unlock(&nbd_index_mutex);
+
+		/* Wait untill the recv_work exit */
+		wait_for_completion(&nbd->complete);
+
+		kfree(nbd->config);
+		nbd->config = NULL;
+
 		nbd_dev_remove(nbd);
 	}
 }
@@ -1134,8 +1144,6 @@ static void nbd_config_put(struct nbd_device *nbd)
 			}
 			kfree(config->socks);
 		}
-		kfree(nbd->config);
-		nbd->config = NULL;
 
 		nbd->tag_set.timeout = 0;
 		nbd->disk->queue->limits.discard_granularity = 0;
@@ -1143,6 +1151,8 @@ static void nbd_config_put(struct nbd_device *nbd)
 		blk_queue_max_discard_sectors(nbd->disk->queue, UINT_MAX);
 		blk_queue_flag_clear(QUEUE_FLAG_DISCARD, nbd->disk->queue);
 
+		complete(&nbd->complete);
+
 		mutex_unlock(&nbd->config_lock);
 		nbd_put(nbd);
 		module_put(THIS_MODULE);
@@ -1596,6 +1606,7 @@ static int nbd_dev_add(int index)
 	nbd->tag_set.flags = BLK_MQ_F_SHOULD_MERGE |
 		BLK_MQ_F_BLOCKING;
 	nbd->tag_set.driver_data = nbd;
+	init_completion(&nbd->complete);
 
 	err = blk_mq_alloc_tag_set(&nbd->tag_set);
 	if (err)
-- 
2.21.0