nbd: Fix use-after-free in pid_show

I got issue as follows:
[  263.886511] BUG: KASAN: use-after-free in pid_show+0x11f/0x13f
[  263.888359] Read of size 4 at addr ffff8880bf0648c0 by task cat/746
[  263.890479] CPU: 0 PID: 746 Comm: cat Not tainted 4.19.90-dirty #140
[  263.893162] Call Trace:
[  263.893509]  dump_stack+0x108/0x15f
[  263.893999]  print_address_description+0xa5/0x372
[  263.894641]  kasan_report.cold+0x236/0x2a8
[  263.895696]  __asan_report_load4_noabort+0x25/0x30
[  263.896365]  pid_show+0x11f/0x13f
[  263.897422]  dev_attr_show+0x48/0x90
[  263.898361]  sysfs_kf_seq_show+0x24d/0x4b0
[  263.899479]  kernfs_seq_show+0x14e/0x1b0
[  263.900029]  seq_read+0x43f/0x1150
[  263.900499]  kernfs_fop_read+0xc7/0x5a0
[  263.903764]  vfs_read+0x113/0x350
[  263.904231]  ksys_read+0x103/0x270
[  263.905230]  __x64_sys_read+0x77/0xc0
[  263.906284]  do_syscall_64+0x106/0x360
[  263.906797]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reproduce this issue as follows:
1. nbd-server 8000 /tmp/disk
2. nbd-client localhost 8000 /dev/nbd1
3. cat /sys/block/nbd1/pid
Then trigger use-after-free in pid_show.

Reason is after do step '2', nbd-client progress is already exit. So
it's task_struct already freed.
To solve this issue, revert part of 6521d39a's modify and remove
useless 'recv_task' member of nbd_device.

Fixes: 6521d39a ("nbd: Remove variable 'pid'")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20211020073959.2679255-1-yebin10@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -122,10 +122,10 @@ struct nbd_device {
 	struct work_struct remove_work;
 
 	struct list_head list;
-	struct task_struct *task_recv;
 	struct task_struct *task_setup;
 
 	unsigned long flags;
+	pid_t pid; /* pid of nbd-client, if attached */
 
 	char *backend;
 };
@@ -223,7 +223,7 @@ static ssize_t pid_show(struct device *dev,
 	struct gendisk *disk = dev_to_disk(dev);
 	struct nbd_device *nbd = (struct nbd_device *)disk->private_data;
 
-	return sprintf(buf, "%d\n", task_pid_nr(nbd->task_recv));
+	return sprintf(buf, "%d\n", nbd->pid);
 }
 
 static const struct device_attribute pid_attr = {
@@ -335,7 +335,7 @@ static int nbd_set_size(struct nbd_device *nbd, loff_t bytesize,
 	nbd->config->bytesize = bytesize;
 	nbd->config->blksize_bits = __ffs(blksize);
 
-	if (!nbd->task_recv)
+	if (!nbd->pid)
 		return 0;
 
 	if (nbd->config->flags & NBD_FLAG_SEND_TRIM) {
@@ -1300,7 +1300,7 @@ static void nbd_config_put(struct nbd_device *nbd)
 		if (test_and_clear_bit(NBD_RT_HAS_PID_FILE,
 				       &config->runtime_flags))
 			device_remove_file(disk_to_dev(nbd->disk), &pid_attr);
-		nbd->task_recv = NULL;
+		nbd->pid = 0;
 		if (test_and_clear_bit(NBD_RT_HAS_BACKEND_FILE,
 				       &config->runtime_flags)) {
 			device_remove_file(disk_to_dev(nbd->disk), &backend_attr);
@@ -1341,7 +1341,7 @@ static int nbd_start_device(struct nbd_device *nbd)
 	int num_connections = config->num_connections;
 	int error = 0, i;
 
-	if (nbd->task_recv)
+	if (nbd->pid)
 		return -EBUSY;
 	if (!config->socks)
 		return -EINVAL;
@@ -1360,7 +1360,7 @@ static int nbd_start_device(struct nbd_device *nbd)
 	}
 
 	blk_mq_update_nr_hw_queues(&nbd->tag_set, config->num_connections);
-	nbd->task_recv = current;
+	nbd->pid = task_pid_nr(current);
 
 	nbd_parse_flags(nbd);
 
@@ -1616,8 +1616,8 @@ static int nbd_dbg_tasks_show(struct seq_file *s, void *unused)
 {
 	struct nbd_device *nbd = s->private;
 
-	if (nbd->task_recv)
-		seq_printf(s, "recv: %d\n", task_pid_nr(nbd->task_recv));
+	if (nbd->pid)
+		seq_printf(s, "recv: %d\n", nbd->pid);
 
 	return 0;
 }
@@ -2198,7 +2198,7 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
 	mutex_lock(&nbd->config_lock);
 	config = nbd->config;
 	if (!test_bit(NBD_RT_BOUND, &config->runtime_flags) ||
-	    !nbd->task_recv) {
+	    !nbd->pid) {
 		dev_err(nbd_to_dev(nbd),
 			"not configured, cannot reconfigure\n");
 		ret = -EINVAL;