netdev - [PATCH net-next 1/3] ip: discard IPv4 datagrams with overlapping segments.

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Message-Id: <20180802224600.43070-2-posk@google.com>
Date:   Thu,  2 Aug 2018 22:45:58 +0000
From:   Peter Oskolkov <posk@...gle.com>
To:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Florian Westphal <fw@...len.de>,
        Peter Oskolkov <posk@...gle.com>
Subject: [PATCH net-next 1/3] 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.

Suggested-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Peter Oskolkov <posk@...gle.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Florian Westphal <fw@...len.de>
---
 include/uapi/linux/snmp.h |  1 +
 net/ipv4/ip_fragment.c    | 75 ++++++++++-----------------------------
 net/ipv4/proc.c           |  1 +
 3 files changed, 21 insertions(+), 56 deletions(-)

diff --git a/include/uapi/linux/snmp.h b/include/uapi/linux/snmp.h
index e5ebc83827ab..da1a144f1a51 100644
--- a/include/uapi/linux/snmp.h
+++ b/include/uapi/linux/snmp.h
@@ -40,6 +40,7 @@ enum
 	IPSTATS_MIB_REASMREQDS,			/* ReasmReqds */
 	IPSTATS_MIB_REASMOKS,			/* ReasmOKs */
 	IPSTATS_MIB_REASMFAILS,			/* ReasmFails */
+	IPSTATS_MIB_REASM_OVERLAPS,		/* ReasmOverlaps */
 	IPSTATS_MIB_FRAGOKS,			/* FragOKs */
 	IPSTATS_MIB_FRAGFAILS,			/* FragFails */
 	IPSTATS_MIB_FRAGCREATES,		/* FragCreates */
diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index d14d741fb05e..960bf5eab59f 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -277,6 +277,7 @@ 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;
@@ -357,65 +358,23 @@ 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;
@@ -463,6 +422,10 @@ static int ip_frag_queue(struct ipq *qp, struct sk_buff *skb)
 	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 b46e4cf9a55a..70289682a670 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -119,6 +119,7 @@ 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
 };
 
-- 
2.18.0.597.ga71716f1ad-goog