[PATCH net] tipc: Guard against tiny MTU in tipc_msg_build()

Qian Zhang (张谦) reported a potential socket buffer overflow in
tipc_msg_build().  The minimum fragment length needs to be checked
against the maximum packet size, which is based on the link MTU.

Reported-by: Qian Zhang (张谦) <redacted>
Signed-off-by: Ben Hutchings <redacted>
---
This is untested, but I think it fixes the issue reported.  Ideally
tipc_l2_device_event() would also disable use of TIPC on devices with
too small an MTU, like several other protocols do.

Ben.

 net/tipc/msg.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/tipc/msg.c b/net/tipc/msg.c
index 17201aa8423d..b9124ac82c29 100644
--- a/net/tipc/msg.c
+++ b/net/tipc/msg.c

@@ -274,6 +274,10 @@ int tipc_msg_build(struct tipc_msg *mhdr, struct msghdr *m,
 		goto error;
 	}
 
+	/* Check that fragment and message header will fit */
+	if (INT_H_SIZE + mhsz > pktmax)
+		return -EMSGSIZE;
+
 	/* Prepare reusable fragment header */
 	tipc_msg_init(msg_prevnode(mhdr), &pkthdr, MSG_FRAGMENTER,
 		      FIRST_FRAGMENT, INT_H_SIZE, msg_destnode(mhdr));