Index: sys/netinet/tcp_output.c
===================================================================
--- sys/netinet/tcp_output.c
+++ sys/netinet/tcp_output.c
@@ -60,6 +60,7 @@
 #include <sys/socket.h>
 #include <sys/socketvar.h>
 #include <sys/sysctl.h>
+#include <sys/syslog.h>
 #include <sys/stats.h>
 
 #include <net/if.h>
@@ -1569,6 +1570,15 @@
 		tp->snd_nxt += len;
 		if (SEQ_GT(tp->snd_nxt, tp->snd_max)) {
 			tp->snd_max = tp->snd_nxt;
+			struct socket *so = tp->t_inpcb->inp_socket;
+			tcp_seq top = tp->snd_una + sbused(&so->so_snd);
+			KASSERT(SEQ_LEQ(tp->snd_max, top+1),
+				("%s: snd_max beyond so_snd", __func__));
+			if (SEQ_GT(tp->snd_max, top + 1)) {
+				log(LOG_CRIT,"%s#%d: snd_max %u > so_snd+1 %u adjusting.\n",
+					__func__, __LINE__, tp->snd_max, top + 1);
+				tp->snd_max = top + 1;
+			}
 			/*
 			 * Time this transmission if not a retransmission and
 			 * not currently timing anything.
@@ -1644,8 +1654,18 @@
 			++xlen;
 			tp->t_flags |= TF_SENTFIN;
 		}
-		if (SEQ_GT(tp->snd_nxt + xlen, tp->snd_max))
+		if (SEQ_GT(tp->snd_nxt + xlen, tp->snd_max)) {
 			tp->snd_max = tp->snd_nxt + xlen;
+			struct socket *so = tp->t_inpcb->inp_socket;
+			tcp_seq top = tp->snd_una + sbused(&so->so_snd);
+			KASSERT(SEQ_LEQ(tp->snd_max, top + 1),
+			    ("%s: snd_max beyond so_snd", __func__));
+			if (SEQ_GT(tp->snd_max, top + 1)) {
+				log(LOG_CRIT,"%s#%d: snd_max %u > so_snd+1 %u adjusting.\n",
+					__func__, __LINE__, tp->snd_max, top + 1);
+				tp->snd_max = top + 1;
+			}
+		}
 	}
 	if ((error == 0) &&
 	    (TCPS_HAVEESTABLISHED(tp->t_state) &&
Index: sys/netinet/tcp_sack.c
===================================================================
--- sys/netinet/tcp_sack.c
+++ sys/netinet/tcp_sack.c
@@ -898,8 +898,20 @@
 			highdata--;
 		if (th->th_ack != highdata) {
 			tp->snd_fack = th->th_ack;
-			(void)tcp_sackhole_insert(tp, SEQ_MAX(th->th_ack,
-			    highdata - maxseg), highdata, NULL);
+			struct socket *so = tp->t_inpcb->inp_socket;
+			tcp_seq top = tp->snd_una + sbused(&so->so_snd);
+			KASSERT(SEQ_LEQ(highdata, top),
+				("%s: rescue.end > so_snd, NEEDFIN:%d SENTFIN:%d\n",
+				__func__, (tp->t_flags & TF_NEEDFIN) != 0,
+				(tp->t_flags & TF_SENTFIN) != 0));
+			if (SEQ_GT(highdata, top)) {
+				log(LOG_CRIT,"%s#%d: rescue (%u-%u) > so_snd %u skipping.\n",
+					__func__, __LINE__,
+					SEQ_MAX(th->th_ack, highdata - maxseg),
+					highdata, top);
+			} else
+				(void)tcp_sackhole_insert(tp, SEQ_MAX(th->th_ack,
+				    highdata - maxseg), highdata, NULL);
 		}
 	}
 	(void) tcp_output(tp);
@@ -969,17 +981,19 @@
 			}
 		}
 	}
-	KASSERT(SEQ_LT(hole->start, hole->end), ("%s: hole.start >= hole.end", __func__));
-	if (!(V_tcp_do_newsack)) {
-		KASSERT(SEQ_LT(hole->start, tp->snd_fack), ("%s: hole.start >= snd.fack", __func__));
-		KASSERT(SEQ_LT(hole->end, tp->snd_fack), ("%s: hole.end >= snd.fack", __func__));
-		KASSERT(SEQ_LT(hole->rxmit, tp->snd_fack), ("%s: hole.rxmit >= snd.fack", __func__));
+	{
+		struct socket *so = tp->t_inpcb->inp_socket;
+		tcp_seq top = tp->snd_una + sbused(&so->so_snd);
+		KASSERT(SEQ_LT(hole->start, hole->end), ("%s: hole.start >= hole.end", __func__));
+		KASSERT(SEQ_LEQ(hole->start, top), ("%s: hole.start >= so_snd", __func__));
+		KASSERT(SEQ_LEQ(hole->end,   top), ("%s: hole.end >= so_snd", __func__));
+		KASSERT(SEQ_LEQ(hole->rxmit, top), ("%s: hole.rxmit >= so_snd", __func__));
 		if (SEQ_GEQ(hole->start, hole->end) ||
-		    SEQ_GEQ(hole->start, tp->snd_fack) ||
-		    SEQ_GEQ(hole->end, tp->snd_fack) ||
-		    SEQ_GEQ(hole->rxmit, tp->snd_fack)) {
-			log(LOG_CRIT,"tcp: invalid SACK hole (%u-%u,%u) vs fwd ack %u, ignoring.\n",
-					hole->start, hole->end, hole->rxmit, tp->snd_fack);
+		    SEQ_GT(hole->start, top) ||
+		    SEQ_GT(hole->end,   top) ||
+		    SEQ_GT(hole->rxmit, top)) {
+			log(LOG_CRIT,"%s#%d: invalid SACK hole (%u-%u,%u) vs so_snd %u ignoring.\n",
+					__func__, __LINE__, hole->start, hole->end, hole->rxmit, top);
 			return (NULL);
 		}
 	}