[PATCH] net: phonet: do not BUG_ON() in pn_socket_autobind() on failed bind

[Morduan Zang]

syzbot reported a kernel BUG triggered from pn_socket_sendmsg() via
pn_socket_autobind():

  kernel BUG at net/phonet/socket.c:213!
  RIP: 0010:pn_socket_autobind net/phonet/socket.c:213 [inline]
  RIP: 0010:pn_socket_sendmsg+0x240/0x250 net/phonet/socket.c:421
  Call Trace:
   sock_sendmsg_nosec+0x112/0x150 net/socket.c:797
   __sock_sendmsg net/socket.c:812 [inline]
   __sys_sendto+0x402/0x590 net/socket.c:2280
   ...

pn_socket_autobind() calls pn_socket_bind() with port 0 and, on
-EINVAL, assumes the socket was already bound and asserts that the
port is non-zero:

  err = pn_socket_bind(sock, ..., sizeof(struct sockaddr_pn));
  if (err != -EINVAL)
          return err;
  BUG_ON(!pn_port(pn_sk(sock->sk)->sobject));
  return 0; /* socket was already bound */

However pn_socket_bind() also returns -EINVAL when sk->sk_state is not
TCP_CLOSE, even when the socket has never been bound and pn_port() is
still 0.  In that case the BUG_ON() fires and panics the kernel from a
user-triggerable path.

Treat the "bind returned -EINVAL but pn_port() is still 0" case as a
regular error and propagate -EINVAL to the caller instead of crashing.
Existing callers already translate a non-zero return from
pn_socket_autobind() into -ENOBUFS/-EAGAIN, so returning -EINVAL here
only changes behaviour from panic to a normal errno.

Fixes: ba113a94b750 ("Phonet: common socket glue")
Reported-by: syzbot+706f5eb79044e686c794@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=706f5eb79044e686c794
Signed-off-by: Morduan Zang <zhangdandan@xxxxxxxxxxxxx>
Signed-off-by: zhanjun <zhanjun@xxxxxxxxxxxxx>
---
 net/phonet/socket.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/net/phonet/socket.c b/net/phonet/socket.c
index 4423d483c630..de9108adfe1c 100644
--- a/net/phonet/socket.c
+++ b/net/phonet/socket.c
@@ -210,7 +210,15 @@ static int pn_socket_autobind(struct socket *sock)
 			     sizeof(struct sockaddr_pn));
 	if (err != -EINVAL)
 		return err;
-	BUG_ON(!pn_port(pn_sk(sock->sk)->sobject));
+	/*
+	 * pn_socket_bind() can return -EINVAL both when the socket is
+	 * already bound (pn_port() != 0) and when sk_state != TCP_CLOSE
+	 * without a prior bind.  Only the former is an "already bound"
+	 * success for autobind; otherwise propagate -EINVAL instead of
+	 * crashing the kernel.
+	 */
+	if (!pn_port(pn_sk(sock->sk)->sobject))
+		return -EINVAL;
 	return 0; /* socket was already bound */
 }
 
-- 
2.50.1