stream: check if ACK packet is outdated

Outdated packets are ACK packets w/o data that have an ACK value
lower than our last_ack and also don't have an SACK records that
are new.

This can happen when some packets come in later than others (possibly
due to different paths taken).

src/stream-tcp-sack.c

@@ -296,6 +296,85 @@ int StreamTcpSackUpdatePacket(TcpStream *stream, Packet *p)
     SCReturnInt(0);
 }
 
+static inline int CompareOverlap(
+        struct StreamTcpSackRecord *lookup, struct StreamTcpSackRecord *intree)
+{
+    if (lookup->re <= intree->le) // entirely before
+        return -1;
+    else if (lookup->re >= intree->le && lookup->le < intree->re) // (some) overlap
+        return 0;
+    else
+        return 1; // entirely after
+}
+
+static struct StreamTcpSackRecord *FindOverlap(
+        struct TCPSACK *head, struct StreamTcpSackRecord *elm)
+{
+    SCLogDebug("looking up le:%u re:%u", elm->le, elm->re);
+
+    struct StreamTcpSackRecord *tmp = RB_ROOT(head);
+    struct StreamTcpSackRecord *res = NULL;
+    while (tmp) {
+        SCLogDebug("compare with le:%u re:%u", tmp->le, tmp->re);
+        const int comp = CompareOverlap(elm, tmp);
+        SCLogDebug("compare result: %d", comp);
+        if (comp < 0) {
+            res = tmp;
+            tmp = RB_LEFT(tmp, rb);
+        } else if (comp > 0) {
+            tmp = RB_RIGHT(tmp, rb);
+        } else {
+            return tmp;
+        }
+    }
+    return res;
+}
+
+bool StreamTcpSackPacketIsOutdated(TcpStream *stream, Packet *p)
+{
+    const int records = TCP_GET_SACK_CNT(p);
+    const uint8_t *data = TCP_GET_SACK_PTR(p);
+    if (records > 0 && data != NULL) {
+        int sack_outdated = 0;
+        TCPOptSackRecord rec[records], *sack_rec = rec;
+        memcpy(&rec, data, sizeof(TCPOptSackRecord) * records);
+        for (int record = 0; record < records; record++) {
+            const uint32_t le = SCNtohl(sack_rec->le);
+            const uint32_t re = SCNtohl(sack_rec->re);
+            SCLogDebug("%p last_ack %u, left edge %u, right edge %u", sack_rec, stream->last_ack,
+                    le, re);
+
+            struct StreamTcpSackRecord lookup = { .le = le, .re = re };
+            struct StreamTcpSackRecord *res = FindOverlap(&stream->sack_tree, &lookup);
+            SCLogDebug("res %p", res);
+            if (res) {
+                if (le >= res->le && re <= res->re) {
+                    SCLogDebug("SACK rec le:%u re:%u eclipsed by in tree le:%u re:%u", le, re,
+                            res->le, res->re);
+                    sack_outdated++;
+                } else {
+                    SCLogDebug("SACK rec le:%u re:%u SACKs new DATA vs in tree le:%u re:%u", le, re,
+                            res->le, res->re);
+                }
+            } else {
+                SCLogDebug("SACK rec le:%u re:%u SACKs new DATA. No match in tree", le, re);
+            }
+            sack_rec++;
+        }
+#ifdef DEBUG
+        StreamTcpSackPrintList(stream);
+#endif
+        if (records != sack_outdated) {
+            // SACK tree needs updating
+            return false;
+        } else {
+            // SACK list is packet is completely outdated
+            return true;
+        }
+    }
+    return false;
+}
+
 void StreamTcpSackPruneList(TcpStream *stream)
 {
     SCEnter();

src/stream-tcp-sack.h

@@ -39,6 +39,7 @@ static inline uint32_t StreamTcpSackedSize(TcpStream *stream)
 }
 
 int StreamTcpSackUpdatePacket(TcpStream *, Packet *);
+bool StreamTcpSackPacketIsOutdated(TcpStream *stream, Packet *p);
 void StreamTcpSackPruneList(TcpStream *);
 void StreamTcpSackFreeList(TcpStream *);
 void StreamTcpSackRegisterTests (void);

src/stream-tcp.c

@@ -2510,6 +2510,66 @@ static inline uint32_t StreamTcpResetGetMaxAck(TcpStream *stream, uint32_t seq)
     SCReturnUInt(ack);
 }
 
+/** \internal
+ *  \brief check if a ACK packet is outdated so processing can be fast tracked
+ *
+ *  Consider a packet outdated ack if:
+ *  - state is >= ESTABLISHED
+ *  - ACK < last_ACK
+ *  - SACK acks nothing new
+ *  - packet has no data
+ *  - SEQ == next_SEQ
+ *  - flags has ACK set but don't contain SYN/FIN/RST
+ *
+ *  \todo the most likely explanation for this packet is that we already
+ *        accepted a "newer" ACK. We will not consider an outdated timestamp
+ *        option an issue for this packet, but we should probably still
+ *        check if the ts isn't too far off.
+ */
+static bool StreamTcpPacketIsOutdatedAck(TcpSession *ssn, Packet *p)
+{
+    if (ssn->state < TCP_ESTABLISHED)
+        return false;
+    if (p->payload_len != 0)
+        return false;
+    if ((p->tcph->th_flags & (TH_ACK | TH_SYN | TH_FIN | TH_RST)) != TH_ACK)
+        return false;
+
+    /* lets see if this is a packet that is entirely eclipsed by earlier ACKs */
+    if (PKT_IS_TOSERVER(p)) {
+        if (SEQ_EQ(TCP_GET_SEQ(p), ssn->client.next_seq) &&
+                SEQ_LT(TCP_GET_ACK(p), ssn->server.last_ack)) {
+            if (!TCP_HAS_SACK(p)) {
+                SCLogDebug("outdated ACK (no SACK, SEQ %u vs next_seq %u)", TCP_GET_SEQ(p),
+                        ssn->client.next_seq);
+                return true;
+            }
+
+            if (StreamTcpSackPacketIsOutdated(&ssn->server, p)) {
+                SCLogDebug("outdated ACK (have SACK, SEQ %u vs next_seq %u)", TCP_GET_SEQ(p),
+                        ssn->client.next_seq);
+                return true;
+            }
+        }
+    } else {
+        if (SEQ_EQ(TCP_GET_SEQ(p), ssn->server.next_seq) &&
+                SEQ_LT(TCP_GET_ACK(p), ssn->client.last_ack)) {
+            if (!TCP_HAS_SACK(p)) {
+                SCLogDebug("outdated ACK (no SACK, SEQ %u vs next_seq %u)", TCP_GET_SEQ(p),
+                        ssn->client.next_seq);
+                return true;
+            }
+
+            if (StreamTcpSackPacketIsOutdated(&ssn->client, p)) {
+                SCLogDebug("outdated ACK (have SACK, SEQ %u vs next_seq %u)", TCP_GET_SEQ(p),
+                        ssn->client.next_seq);
+                return true;
+            }
+        }
+    }
+    return false;
+}
+
 /**
  *  \brief  Function to handle the TCP_ESTABLISHED state. The function handles
  *          ACK, FIN, RST packets and correspondingly changes the connection
@@ -4893,10 +4953,14 @@ int StreamTcpPacket (ThreadVars *tv, Packet *p, StreamTcpThread *stt,
 
         /* if packet is not a valid window update, check if it is perhaps
          * a bad window update that we should ignore (and alert on) */
-        if (StreamTcpPacketIsFinShutdownAck(ssn, p) == 0)
+        if (StreamTcpPacketIsFinShutdownAck(ssn, p) == 0) {
-            if (StreamTcpPacketIsWindowUpdate(ssn, p) == 0)
+            if (StreamTcpPacketIsWindowUpdate(ssn, p) == 0) {
                 if (StreamTcpPacketIsBadWindowUpdate(ssn,p))
                     goto skip;
+                if (StreamTcpPacketIsOutdatedAck(ssn, p))
+                    goto skip;
+            }
+        }
 
         /* handle the per 'state' logic */
         if (StreamTcpStateDispatch(tv, p, stt, ssn, &stt->pseudo_queue, ssn->state) < 0)