net: qrtr: Fix memory leak in qrtr_tun_open

If qrtr_endpoint_register() failed, tun is leaked.
Fix this, by freeing tun in error path.

syzbot report:
BUG: memory leak
unreferenced object 0xffff88811848d680 (size 64):
  comm "syz-executor684", pid 10171, jiffies 4294951561 (age 26.070s)
  hex dump (first 32 bytes):
    80 dd 0a 84 ff ff ff ff 00 00 00 00 00 00 00 00  ................
    90 d6 48 18 81 88 ff ff 90 d6 48 18 81 88 ff ff  ..H.......H.....
  backtrace:
    [<0000000018992a50>] kmalloc include/linux/slab.h:552 [inline]
    [<0000000018992a50>] kzalloc include/linux/slab.h:682 [inline]
    [<0000000018992a50>] qrtr_tun_open+0x22/0x90 net/qrtr/tun.c:35
    [<0000000003a453ef>] misc_open+0x19c/0x1e0 drivers/char/misc.c:141
    [<00000000dec38ac8>] chrdev_open+0x10d/0x340 fs/char_dev.c:414
    [<0000000079094996>] do_dentry_open+0x1e6/0x620 fs/open.c:817
    [<000000004096d290>] do_open fs/namei.c:3252 [inline]
    [<000000004096d290>] path_openat+0x74a/0x1b00 fs/namei.c:3369
    [<00000000b8e64241>] do_filp_open+0xa0/0x190 fs/namei.c:3396
    [<00000000a3299422>] do_sys_openat2+0xed/0x230 fs/open.c:1172
    [<000000002c1bdcef>] do_sys_open fs/open.c:1188 [inline]
    [<000000002c1bdcef>] __do_sys_openat fs/open.c:1204 [inline]
    [<000000002c1bdcef>] __se_sys_openat fs/open.c:1199 [inline]
    [<000000002c1bdcef>] __x64_sys_openat+0x7f/0xe0 fs/open.c:1199
    [<00000000f3a5728f>] do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
    [<000000004b38b7ec>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 28fb4e59 ("net: qrtr: Expose tunneling endpoint to user space")
Reported-by: syzbot+5d6e4af21385f5cfc56a@syzkaller.appspotmail.com
Signed-off-by: Takeshi Misawa <jeliantsurux@gmail.com>
Link: https://lore.kernel.org/r/20210221234427.GA2140@DESKTOPSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/qrtr/tun.c

@@ -31,6 +31,7 @@ static int qrtr_tun_send(struct qrtr_endpoint *ep, struct sk_buff *skb)
 static int qrtr_tun_open(struct inode *inode, struct file *filp)
 {
 	struct qrtr_tun *tun;
+	int ret;
 
 	tun = kzalloc(sizeof(*tun), GFP_KERNEL);
 	if (!tun)
@@ -43,7 +44,16 @@ static int qrtr_tun_open(struct inode *inode, struct file *filp)
 
 	filp->private_data = tun;
 
-	return qrtr_endpoint_register(&tun->ep, QRTR_EP_NID_AUTO);
+	ret = qrtr_endpoint_register(&tun->ep, QRTR_EP_NID_AUTO);
+	if (ret)
+		goto out;
+
+	return 0;
+
+out:
+	filp->private_data = NULL;
+	kfree(tun);
+	return ret;
 }
 
 static ssize_t qrtr_tun_read_iter(struct kiocb *iocb, struct iov_iter *to)