af_unix: read UNIX_DIAG_VFS data under unix_state_lock

Exact UNIX diag lookups hold a reference to the socket, but not to
u->path. Meanwhile, unix_release_sock() clears u->path under
unix_state_lock() and drops the path reference after unlocking.

Read the inode and device numbers for UNIX_DIAG_VFS while holding
unix_state_lock(), then emit the netlink attribute after dropping the
lock.

This keeps the VFS data stable while the reply is being built.

Fixes: 5f7b056 ("unix_diag: Unix inode info NLA")
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Co-developed-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Yuan Tan <yuantan098@gmail.com>
Suggested-by: Xin Liu <bird@lzu.edu.cn>
Tested-by: Ren Wei <enjou1224z@gmail.com>
Signed-off-by: Jiexun Wang <wangjiexun2025@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260407080015.1744197-1-n05ec@lzu.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Jiexun Wang

net/unix/diag.c

@@ -28,18 +28,23 @@ static int sk_diag_dump_name(struct sock *sk, struct sk_buff *nlskb)
 
 static int sk_diag_dump_vfs(struct sock *sk, struct sk_buff *nlskb)
 {
-	struct dentry *dentry = unix_sk(sk)->path.dentry;
+	struct unix_diag_vfs uv;
+	struct dentry *dentry;
+	bool have_vfs = false;
 
+	unix_state_lock(sk);
+	dentry = unix_sk(sk)->path.dentry;
 	if (dentry) {
-		struct unix_diag_vfs uv = {
-			.udiag_vfs_ino = d_backing_inode(dentry)->i_ino,
-			.udiag_vfs_dev = dentry->d_sb->s_dev,
-		};
-
-		return nla_put(nlskb, UNIX_DIAG_VFS, sizeof(uv), &uv);
+		uv.udiag_vfs_ino = d_backing_inode(dentry)->i_ino;
+		uv.udiag_vfs_dev = dentry->d_sb->s_dev;
+		have_vfs = true;
 	}
+	unix_state_unlock(sk);
 
-	return 0;
+	if (!have_vfs)
+		return 0;
+
+	return nla_put(nlskb, UNIX_DIAG_VFS, sizeof(uv), &uv);
 }
 
 static int sk_diag_dump_peer(struct sock *sk, struct sk_buff *nlskb)