ip: discard IPv4 datagrams with overlapping segments.

This behavior is required in IPv6, and there is little need
to tolerate overlapping fragments in IPv4. This change
simplifies the code and eliminates potential DDoS attack vectors.

Tested: ran ip_defrag selftest (not yet available uptream).

Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Florian Westphal <fw@strlen.de>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 7969e5c40dfd04799d4341f1b7cd266b6e47f227)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 21 insertions, 56 deletions

diff --git a/include/uapi/linux/snmp.h b/include/uapi/linux/snmp.h
index e7a31f830690..3442a26d36d9 100644
--- a/include/uapi/linux/snmp.h
+++ b/include/uapi/linux/snmp.h
@@ -55,6 +55,7 @@ enum
 	IPSTATS_MIB_ECT1PKTS,			/* InECT1Pkts */
 	IPSTATS_MIB_ECT0PKTS,			/* InECT0Pkts */
 	IPSTATS_MIB_CEPKTS,			/* InCEPkts */
+	IPSTATS_MIB_REASM_OVERLAPS,		/* ReasmOverlaps */
 	__IPSTATS_MIB_MAX
 };
 
diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 73c0adc61a65..8bfb34e9ea32 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -275,6 +275,7 @@ static int ip_frag_reinit(struct ipq *qp)
 /* Add new segment to existing queue. */
 static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
 {
+	struct net *net = container_of(qp->q.net, struct net, ipv4.frags);
 	struct sk_buff *prev, *next;
 	struct net_device *dev;
 	unsigned int fragsize;
@@ -355,65 +356,23 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
 	}
 
 found:
-	/* We found where to put this one.  Check for overlap with
-	 * preceding fragment, and, if needed, align things so that
-	 * any overlaps are eliminated.
+	/* RFC5722, Section 4, amended by Errata ID : 3089
+	 *                          When reassembling an IPv6 datagram, if
+	 *   one or more its constituent fragments is determined to be an
+	 *   overlapping fragment, the entire datagram (and any constituent
+	 *   fragments) MUST be silently discarded.
+	 *
+	 * We do the same here for IPv4.
 	 */
-	if (prev) {
-		int i = (prev->ip_defrag_offset + prev->len) - offset;
 
-		if (i > 0) {
-			offset += i;
-			err = -EINVAL;
-			if (end <= offset)
-				goto err;
-			err = -ENOMEM;
-			if (!pskb_pull(skb, i))
-				goto err;
-			if (skb->ip_summed != CHECKSUM_UNNECESSARY)
-				skb->ip_summed = CHECKSUM_NONE;
-		}
-	}
+	/* Is there an overlap with the previous fragment? */
+	if (prev &&
+	    (prev->ip_defrag_offset + prev->len) > offset)
+		goto discard_qp;
 
-	err = -ENOMEM;
-
-	while (next && next->ip_defrag_offset < end) {
-		int i = end - next->ip_defrag_offset; /* overlap is 'i' bytes */
-
-		if (i < next->len) {
-			int delta = -next->truesize;
-
-			/* Eat head of the next overlapped fragment
-			 * and leave the loop. The next ones cannot overlap.
-			 */
-			if (!pskb_pull(next, i))
-				goto err;
-			delta += next->truesize;
-			if (delta)
-				add_frag_mem_limit(qp->q.net, delta);
-			next->ip_defrag_offset += i;
-			qp->q.meat -= i;
-			if (next->ip_summed != CHECKSUM_UNNECESSARY)
-				next->ip_summed = CHECKSUM_NONE;
-			break;
-		} else {
-			struct sk_buff *free_it = next;
-
-			/* Old fragment is completely overridden with
-			 * new one drop it.
-			 */
-			next = next->next;
-
-			if (prev)
-				prev->next = next;
-			else
-				qp->q.fragments = next;
-
-			qp->q.meat -= free_it->len;
-			sub_frag_mem_limit(qp->q.net, free_it->truesize);
-			kfree_skb(free_it);
-		}
-	}
+	/* Is there an overlap with the next fragment? */
+	if (next && next->ip_defrag_offset < end)
+		goto discard_qp;
 
 	/* Note : skb->ip_defrag_offset and skb->dev share the same location */
 	dev = skb->dev;
@@ -461,6 +420,10 @@ found:
 	skb_dst_drop(skb);
 	return -EINPROGRESS;
 
+discard_qp:
+	inet_frag_kill(&qp->q);
+	err = -EINVAL;
+	__IP_INC_STATS(net, IPSTATS_MIB_REASM_OVERLAPS);
 err:
 	kfree_skb(skb);
 	return err;
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index aa1e52587bf5..ec48d8eafc7e 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -134,6 +134,7 @@ static const struct snmp_mib snmp4_ipextstats_list[] = {
 	SNMP_MIB_ITEM("InECT1Pkts", IPSTATS_MIB_ECT1PKTS),
 	SNMP_MIB_ITEM("InECT0Pkts", IPSTATS_MIB_ECT0PKTS),
 	SNMP_MIB_ITEM("InCEPkts", IPSTATS_MIB_CEPKTS),
+	SNMP_MIB_ITEM("ReasmOverlaps", IPSTATS_MIB_REASM_OVERLAPS),
 	SNMP_MIB_SENTINEL
 };