tomoyo: Don't use nifty names on sockets.

syzbot is reporting that use of SOCKET_I()->sk from open() can result in
use after free problem [1], for socket's inode is still reachable via
/proc/pid/fd/n despite destruction of SOCKET_I()->sk already completed.

At first I thought that this race condition applies to only open/getattr
permission checks. But James Morris has pointed out that there are more
permission checks where this race condition applies to. Thus, get rid of
tomoyo_get_socket_name() instead of conditionally bypassing permission
checks on sockets. As a side effect of this patch,
"socket:[family=\$:type=\$:protocol=\$]" in the policy files has to be
rewritten to "socket:[\$]".

[1] https://syzkaller.appspot.com/bug?id=73d590010454403d55164cca23bd0565b1eb3b74Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+0341f6a4d729d4e0acf1@syzkaller.appspotmail.com>
Reported-by: James Morris <jmorris@namei.org>

security/tomoyo/realpath.c

@@ -217,31 +217,6 @@ static char *tomoyo_get_local_path(struct dentry *dentry, char * const buffer,
 	return ERR_PTR(-ENOMEM);
 }
 
-/**
- * tomoyo_get_socket_name - Get the name of a socket.
- *
- * @path:   Pointer to "struct path".
- * @buffer: Pointer to buffer to return value in.
- * @buflen: Sizeof @buffer.
- *
- * Returns the buffer.
- */
-static char *tomoyo_get_socket_name(const struct path *path, char * const buffer,
-				    const int buflen)
-{
-	struct inode *inode = d_backing_inode(path->dentry);
-	struct socket *sock = inode ? SOCKET_I(inode) : NULL;
-	struct sock *sk = sock ? sock->sk : NULL;
-
-	if (sk) {
-		snprintf(buffer, buflen, "socket:[family=%u:type=%u:protocol=%u]",
-			 sk->sk_family, sk->sk_type, sk->sk_protocol);
-	} else {
-		snprintf(buffer, buflen, "socket:[unknown]");
-	}
-	return buffer;
-}
-
 /**
  * tomoyo_realpath_from_path - Returns realpath(3) of the given pathname but ignores chroot'ed root.
  *
@@ -279,12 +254,7 @@ char *tomoyo_realpath_from_path(const struct path *path)
 			break;
 		/* To make sure that pos is '\0' terminated. */
 		buf[buf_len - 1] = '\0';
-		/* Get better name for socket. */
-		if (sb->s_magic == SOCKFS_MAGIC) {
-			pos = tomoyo_get_socket_name(path, buf, buf_len - 1);
-			goto encode;
-		}
-		/* For "pipe:[\$]". */
+		/* For "pipe:[\$]" and "socket:[\$]". */
 		if (dentry->d_op && dentry->d_op->d_dname) {
 			pos = dentry->d_op->d_dname(dentry, buf, buf_len - 1);
 			goto encode;