diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index eb7f82e..65f6c04 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1222,7 +1222,7 @@
 	}
 
 	/* D-SACK for already forgotten data... Do dumb counting. */
-	if (dup_sack &&
+	if (dup_sack && tp->undo_marker && tp->undo_retrans &&
 	    !after(end_seq_0, prior_snd_una) &&
 	    after(end_seq_0, tp->undo_marker))
 		tp->undo_retrans--;
@@ -1299,7 +1299,8 @@
 
 	/* Account D-SACK for retransmitted packet. */
 	if (dup_sack && (sacked & TCPCB_RETRANS)) {
-		if (after(TCP_SKB_CB(skb)->end_seq, tp->undo_marker))
+		if (tp->undo_marker && tp->undo_retrans &&
+		    after(TCP_SKB_CB(skb)->end_seq, tp->undo_marker))
 			tp->undo_retrans--;
 		if (sacked & TCPCB_SACKED_ACKED)
 			state->reord = min(fack_count, state->reord);
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 406f320..dfa5beb 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2162,7 +2162,7 @@
 		if (!tp->retrans_stamp)
 			tp->retrans_stamp = TCP_SKB_CB(skb)->when;
 
-		tp->undo_retrans++;
+		tp->undo_retrans += tcp_skb_pcount(skb);
 
 		/* snd_nxt is stored to detect loss of retransmitted segment,
 		 * see tcp_input.c tcp_sacktag_write_queue().