debugfs: Fix memleak in debugfs_change_name().

syzbot reported memleak in debugfs_change_name(). [0]

When lookup_noperm_unlocked() fails, new_name is leaked.

Let's fix it by reusing kfree_const() at the end of
debugfs_change_name().

[0]:
BUG: memory leak
unreferenced object 0xffff8881110bb308 (size 8):
  comm "syz.0.17", pid 6090, jiffies 4294942958
  hex dump (first 8 bytes):
    2e 00 00 00 00 00 00 00                          ........
  backtrace (crc ecfc7064):
    kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
    slab_post_alloc_hook mm/slub.c:4953 [inline]
    slab_alloc_node mm/slub.c:5258 [inline]
    __do_kmalloc_node mm/slub.c:5651 [inline]
    __kmalloc_node_track_caller_noprof+0x3b2/0x670 mm/slub.c:5759
    __kmemdup_nul mm/util.c:64 [inline]
    kstrdup+0x3c/0x80 mm/util.c:84
    kstrdup_const+0x63/0x80 mm/util.c:104
    kvasprintf_const+0xca/0x110 lib/kasprintf.c:48
    debugfs_change_name+0xf6/0x5d0 fs/debugfs/inode.c:854
    cfg80211_dev_rename+0xd8/0x110 net/wireless/core.c:149
    nl80211_set_wiphy+0x102/0x1770 net/wireless/nl80211.c:3844
    genl_family_rcv_msg_doit+0x11e/0x190 net/netlink/genetlink.c:1115
    genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
    genl_rcv_msg+0x2fd/0x440 net/netlink/genetlink.c:1210
    netlink_rcv_skb+0x93/0x1d0 net/netlink/af_netlink.c:2550
    genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
    netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
    netlink_unicast+0x3a3/0x4f0 net/netlink/af_netlink.c:1344
    netlink_sendmsg+0x335/0x6b0 net/netlink/af_netlink.c:1894
    sock_sendmsg_nosec net/socket.c:718 [inline]
    __sock_sendmsg net/socket.c:733 [inline]
    ____sys_sendmsg+0x562/0x5a0 net/socket.c:2608
    ___sys_sendmsg+0xc8/0x130 net/socket.c:2662
    __sys_sendmsg+0xc7/0x140 net/socket.c:2694

Fixes: 833d2b3 ("Add start_renaming_two_dentries()")
Reported-by: syzbot+3d7ca9c802c547f8550a@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69369d82.a70a0220.38f243.009f.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251208094551.46184-1-kuniyu@google.com
[ Fix minor typo in commit message. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: q2ven

fs/debugfs/inode.c

@@ -841,8 +841,10 @@ int __printf(2, 3) debugfs_change_name(struct dentry *dentry, const char *fmt, .
 	rd.new_parent = rd.old_parent;
 	rd.flags = RENAME_NOREPLACE;
 	target = lookup_noperm_unlocked(&QSTR(new_name), rd.new_parent);
-	if (IS_ERR(target))
-		return PTR_ERR(target);
+	if (IS_ERR(target)) {
+		error = PTR_ERR(target);
+		goto out_free;
+	}
 
 	error = start_renaming_two_dentries(&rd, dentry, target);
 	if (error) {
@@ -862,6 +864,7 @@ int __printf(2, 3) debugfs_change_name(struct dentry *dentry, const char *fmt, .
 out:
 	dput(rd.old_parent);
 	dput(target);
+out_free:
 	kfree_const(new_name);
 	return error;
 }