[PATCH net-next] net: downgrade BUG_ON EIOCBQUEUED in sock_sendmsg_nosec

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

From: Willem de Bruijn <willemb@google.com>

sock_sendmsg_nosec() runs in syscall context but calls BUG_ON() if
proto_ops->sendmsg() returns -EIOCBQUEUED. Crashing the kernel on an
unexpected return value is dangerous for production workloads.

This risk is real: any signed integer overflow in a sendmsg handler
that returns 0xfffffdef (-529, -EIOCBQUEUED) triggers this BUG_ON. In
one month, two such bugs were reported in PF_PACKET, reachable by
a local user with CAP_NET_RAW in a user namespace:
- Commit 73e594c19b4f ("af_packet: Don't cast tpacket_hdr.tp_len to int
  in tpacket_parse_header().")
- In review ("net/packet: prevent TX_RING byte count overflow")
  https://lore.kernel.org/netdev/20260925124829.6595-1-ilkka.lappetelainen@gmail.com/

Replace the BUG_ON with DEBUG_NET_WARN_ON_ONCE() to retain debug and
syzbot coverage, and return -EIO so that -EIOCBQUEUED does not leak to
userspace or trigger the BUG_ON(ret == -EIOCBQUEUED) in VFS callers of
sock_write_iter() such as new_sync_write().

Failing gracefully is safe because the original reason for the BUG_ON
is gone. It was added in commit 599bd19bdc4c ("fs: don't allow to complete
sync iocbs through aio_complete") and commit d8725c86aeba ("get rid of the
size argument of sock_sendmsg()") to prevent an async completion from
corrupting a caller's on-stack kiocb. Commit 1b784140474e ("net: Remove
iocb argument from sendmsg and recvmsg") removed that on-stack kiocb.
More recently commit fcc77d33a34c ("net: Remove support for AIO on
sockets") removed socket AIO support entirely.

Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 net/socket.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/socket.c b/net/socket.c
index bb0e1e118ef6..d7a2d2195a07 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -800,7 +800,10 @@ static inline int sock_sendmsg_nosec(struct socket *sock, struct msghdr *msg)
 	int ret = INDIRECT_CALL_INET(READ_ONCE(sock->ops)->sendmsg, inet6_sendmsg,
 				     inet_sendmsg, sock, msg,
 				     msg_data_left(msg));
-	BUG_ON(ret == -EIOCBQUEUED);
+	if (unlikely(ret == -EIOCBQUEUED)) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		ret = -EIO;
+	}
 
 	if (trace_sock_send_length_enabled())
 		call_trace_sock_send_length(sock->sk, ret, 0);
-- 
2.56.0.rc1.315.gc6ed9934b7-goog