MFS r352673:

When the RACK stack computes the space for user data in a TCP segment,
it wasn't taking the IP level options into account. This patch fixes this.
In addition, it also corrects a KASSERT and adds protection code to assure
that the IP header chain and the TCP head fit in the first fragment as
required by RFC 7112.

MFS: r353035:

RFC 7112 requires a host to put the complete IP header chain
including the TCP header in the first IP packet.
Enforce this in tcp_output(). In addition make sure that at least
one byte payload fits in the TCP segement to allow making progress.
Without this check, a kernel with INVARIANTS will panic.
This issue was found by running an instance of syzkaller.

Approved by:		re (kib@)
Reviewed by:		rrs@ (r352673), jtl@ (r353035)
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D21665
Differential Revision:	https://reviews.freebsd.org/D21666

sys/netinet/tcp_output.c

@@ -931,6 +931,20 @@ tcp_output(struct tcpcb *tp)
 			if (tp->t_flags & TF_NEEDFIN)
 				sendalot = 1;
 		} else {
+			if (optlen + ipoptlen >= tp->t_maxseg) {
+				/*
+				 * Since we don't have enough space to put
+				 * the IP header chain and the TCP header in
+				 * one packet as required by RFC 7112, don't
+				 * send it. Also ensure that at least one
+				 * byte of the payload can be put into the
+				 * TCP segment.
+				 */
+				SOCKBUF_UNLOCK(&so->so_snd);
+				error = EMSGSIZE;
+				sack_rxmit = 0;
+				goto out;
+			}
 			len = tp->t_maxseg - optlen - ipoptlen;
 			sendalot = 1;
 			if (dont_sendalot)

sys/netinet/tcp_stacks/rack.c

@@ -7872,7 +7872,16 @@ rack_output(struct tcpcb *tp)
 		hdrlen += sizeof(struct udphdr);
 	}
 #endif
-	ipoptlen = 0;
+#ifdef INET6
+	if (isipv6)
+		ipoptlen = ip6_optlen(tp->t_inpcb);
+	else
+#endif
+	if (tp->t_inpcb->inp_options)
+		ipoptlen = tp->t_inpcb->inp_options->m_len -
+		    offsetof(struct ipoption, ipopt_list);
+	else
+		ipoptlen = 0;
 #if defined(IPSEC) || defined(IPSEC_SUPPORT)
 	ipoptlen += ipsec_optlen;
 #endif
@@ -7945,6 +7954,20 @@ rack_output(struct tcpcb *tp)
 				sendalot = 1;
 
 		} else {
+			if (optlen + ipoptlen >= tp->t_maxseg) {
+				/*
+				 * Since we don't have enough space to put
+				 * the IP header chain and the TCP header in
+				 * one packet as required by RFC 7112, don't
+				 * send it. Also ensure that at least one
+				 * byte of the payload can be put into the
+				 * TCP segment.
+				 */
+				SOCKBUF_UNLOCK(&so->so_snd);
+				error = EMSGSIZE;
+				sack_rxmit = 0;
+				goto out;
+			}
 			len = tp->t_maxseg - optlen - ipoptlen;
 			sendalot = 1;
 		}
@@ -8438,15 +8461,9 @@ rack_output(struct tcpcb *tp)
 		m->m_pkthdr.csum_flags |= CSUM_TSO;
 		m->m_pkthdr.tso_segsz = tp->t_maxseg - optlen;
 	}
-#if defined(IPSEC) || defined(IPSEC_SUPPORT)
-	KASSERT(len + hdrlen + ipoptlen - ipsec_optlen == m_length(m, NULL),
-	    ("%s: mbuf chain shorter than expected: %d + %u + %u - %u != %u",
-	    __func__, len, hdrlen, ipoptlen, ipsec_optlen, m_length(m, NULL)));
-#else
-	KASSERT(len + hdrlen + ipoptlen == m_length(m, NULL),
-	    ("%s: mbuf chain shorter than expected: %d + %u + %u != %u",
-	    __func__, len, hdrlen, ipoptlen, m_length(m, NULL)));
-#endif
+	KASSERT(len + hdrlen == m_length(m, NULL),
+	    ("%s: mbuf chain different than expected: %d + %u != %u",
+	    __func__, len, hdrlen, m_length(m, NULL)));
 
 #ifdef TCP_HHOOK
 	/* Run HHOOK_TCP_ESTABLISHED_OUT helper hooks. */