tcp: Sanitize CMSG flags and reserved args in tcp_zerocopy_receive.

Explicitly define reserved field and require it and any subsequent
fields to be zero-valued for now. Additionally, limit the valid CMSG
flags that tcp_zerocopy_receive accepts.

Fixes: 7eeba1706eba ("tcp: Add receive timestamp support for receive zerocopy.")
Signed-off-by: Arjun Roy <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: Soheil Hassas Yeganeh <[e-mail]>
Suggested-by: David Ahern <[e-mail]>
Suggested-by: Leon Romanovsky <[e-mail]>
Suggested-by: Jakub Kicinski <[e-mail]>
Acked-by: Jakub Kicinski <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/tcp.h

···
 	__u64 msg_control; /* ancillary data */
 	__u64 msg_controllen;
 	__u32 msg_flags;
-	/* __u32 hole;  Next we must add >1 u32 otherwise length checks fail. */
+	__u32 reserved; /* set to 0 for now */
 };
 #endif /* _UAPI_LINUX_TCP_H */

net/ipv4/tcp.c

···
 		err);
 }
 
+#define TCP_VALID_ZC_MSG_FLAGS   (TCP_CMSG_TS)
 static void tcp_recv_timestamp(struct msghdr *msg, const struct sock *sk,
 			       struct scm_timestamping_internal *tss);
 static void tcp_zc_finalize_rx_tstamp(struct sock *sk,
···
 			return -EFAULT;
 		if (len < offsetofend(struct tcp_zerocopy_receive, length))
 			return -EINVAL;
-		if (len > sizeof(zc)) {
+		if (unlikely(len > sizeof(zc))) {
+			err = check_zeroed_user(optval + sizeof(zc),
+						len - sizeof(zc));
+			if (err < 1)
+				return err == 0 ? -EINVAL : err;
 			len = sizeof(zc);
 			if (put_user(len, optlen))
 				return -EFAULT;
 		}
 		if (copy_from_user(&zc, optval, len))
 			return -EFAULT;
+		if (zc.reserved)
+			return -EINVAL;
+		if (zc.msg_flags &  ~(TCP_VALID_ZC_MSG_FLAGS))
+			return -EINVAL;
 		lock_sock(sk);
 		err = tcp_zerocopy_receive(sk, &zc, &tss);
 		release_sock(sk);