dccp: fix data-race around dp->dccps_mss_cache

dccp_sendmsg() reads dp->dccps_mss_cache before locking the socket.
Same thing in do_dccp_getsockopt().

Add READ_ONCE()/WRITE_ONCE() annotations,
and change dccp_sendmsg() to check again dccps_mss_cache
after socket is locked.

Fixes: 7c65787 ("[DCCP]: Initial implementation")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230803163021.2958262-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/dccp/output.c

@@ -187,7 +187,7 @@ unsigned int dccp_sync_mss(struct sock *sk, u32 pmtu)
 
 	/* And store cached results */
 	icsk->icsk_pmtu_cookie = pmtu;
-	dp->dccps_mss_cache = cur_mps;
+	WRITE_ONCE(dp->dccps_mss_cache, cur_mps);
 
 	return cur_mps;
 }

net/dccp/proto.c

@@ -630,7 +630,7 @@ static int do_dccp_getsockopt(struct sock *sk, int level, int optname,
 		return dccp_getsockopt_service(sk, len,
 					       (__be32 __user *)optval, optlen);
 	case DCCP_SOCKOPT_GET_CUR_MPS:
-		val = dp->dccps_mss_cache;
+		val = READ_ONCE(dp->dccps_mss_cache);
 		break;
 	case DCCP_SOCKOPT_AVAILABLE_CCIDS:
 		return ccid_getsockopt_builtin_ccids(sk, len, optval, optlen);
@@ -739,7 +739,7 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	trace_dccp_probe(sk, len);
 
-	if (len > dp->dccps_mss_cache)
+	if (len > READ_ONCE(dp->dccps_mss_cache))
 		return -EMSGSIZE;
 
 	lock_sock(sk);
@@ -772,6 +772,12 @@ int dccp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		goto out_discard;
 	}
 
+	/* We need to check dccps_mss_cache after socket is locked. */
+	if (len > dp->dccps_mss_cache) {
+		rc = -EMSGSIZE;
+		goto out_discard;
+	}
+
 	skb_reserve(skb, sk->sk_prot->max_header);
 	rc = memcpy_from_msg(skb_put(skb, len), msg, len);
 	if (rc != 0)