[PATCH net v2] 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
Suggested-by: Remi Denis-Courmont <courmisch@xxxxxxxxx>
Signed-off-by: Morduan Zang <zhangdandan@xxxxxxxxxxxxx>
Signed-off-by: zhanjun <zhanjun@xxxxxxxxxxxxx>
---
Changes in v2:
 - Fold the extra port check into the existing -EINVAL test so that
   autobind now reads as a single compact "not already bound" guard
   using unlikely() (Remi Denis-Courmont).
 - Reword the accompanying comment accordingly; no functional change
   vs. v1 other than the code-style simplification.

v1: https://lore.kernel.org/all/81A6570B633FF6FE+20260422013807.63087-1-zhangdandan@xxxxxxxxxxxxx/
---
 net/phonet/socket.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/phonet/socket.c b/net/phonet/socket.c
index 4423d483c630..bbd710d95b97 100644
--- a/net/phonet/socket.c
+++ b/net/phonet/socket.c
@@ -208,9 +208,15 @@ static int pn_socket_autobind(struct socket *sock)
 	sa.spn_family = AF_PHONET;
 	err = pn_socket_bind(sock, (struct sockaddr_unsized *)&sa,
 			     sizeof(struct sockaddr_pn));
-	if (err != -EINVAL)
+	/*
+	 * pn_socket_bind() also returns -EINVAL when sk_state != TCP_CLOSE
+	 * without a prior bind, so -EINVAL alone is not sufficient to infer
+	 * that the socket was already bound.  Only treat it as "already
+	 * bound" when the port is non-zero; otherwise propagate the error
+	 * instead of crashing the kernel.
+	 */
+	if (err != -EINVAL || unlikely(!pn_port(pn_sk(sock->sk)->sobject)))
 		return err;
-	BUG_ON(!pn_port(pn_sk(sock->sk)->sobject));
 	return 0; /* socket was already bound */
 }
 
-- 
2.50.1