CVE-2012-4444: Insufficient Information (NVD-CWE-noinfo)
value: The ip6_frag_queue function in net/ipv6/reassembly.c in the Linux kernel before 2.6.36 allows remote attackers to bypass intended network restrictions via overlapping IPv6 fragments.
CWE description: There is insufficient information about the issue to classify it; details are unkown or unspecified.
Published: 2012-12-21T11:47Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/ipv6/reassembly.c
Commit date: 2010-09-07 13:57:21 -0700
Commit message: ipv6: discard overlapping fragment

RFC5722 prohibits reassembling fragments when some data overlaps.

Bug spotted by Zhang Zuotao <zuotao.zhang@6wind.com>.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
index 545c4141b755e..64cfef1b0a4c5 100644
--- a/net/ipv6/reassembly.c
+++ b/net/ipv6/reassembly.c
@@ -147,17 +147,10 @@ int ip6_frag_match(struct inet_frag_queue *q, void *a)
 			ipv6_addr_equal(&fq->saddr, arg->src) &&
 			ipv6_addr_equal(&fq->daddr, arg->dst));
 }
 EXPORT_SYMBOL(ip6_frag_match);
 
-/* Memory Tracking Functions. */
-static void frag_kfree_skb(struct netns_frags *nf, struct sk_buff *skb)
-{
-	atomic_sub(skb->truesize, &nf->mem);
-	kfree_skb(skb);
-}
-
 void ip6_frag_init(struct inet_frag_queue *q, void *a)
 {
 	struct frag_queue *fq = container_of(q, struct frag_queue, q);
 	struct ip6_create_arg *arg = a;
 
@@ -344,62 +337,26 @@ static int ip6_frag_queue(struct frag_queue *fq, struct sk_buff *skb,
 			break;	/* bingo! */
 		prev = next;
 	}
 
 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:
+	 *                                  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, including those not yet received) MUST be silently
+	 *   discarded.
 	 */
-	if (prev) {
-		int i = (FRAG6_CB(prev)->offset + prev->len) - offset;
 
-		if (i > 0) {
-			offset += i;
-			if (end <= offset)
-				goto err;
-			if (!pskb_pull(skb, i))
-				goto err;
-			if (skb->ip_summed != CHECKSUM_UNNECESSARY)
-				skb->ip_summed = CHECKSUM_NONE;
-		}
-	}
+	/* Check for overlap with preceding fragment. */
+	if (prev &&
+	    (FRAG6_CB(prev)->offset + prev->len) - offset > 0)
+		goto discard_fq;
 
-	/* Look for overlap with succeeding segments.
-	 * If we can merge fragments, do it.
-	 */
-	while (next && FRAG6_CB(next)->offset < end) {
-		int i = end - FRAG6_CB(next)->offset; /* overlap is 'i' bytes */
-
-		if (i < next->len) {
-			/* Eat head of the next overlapped fragment
-			 * and leave the loop. The next ones cannot overlap.
-			 */
-			if (!pskb_pull(next, i))
-				goto err;
-			FRAG6_CB(next)->offset += i;	/* next fragment */
-			fq->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
-				fq->q.fragments = next;
-
-			fq->q.meat -= free_it->len;
-			frag_kfree_skb(fq->q.net, free_it);
-		}
-	}
+	/* Look for overlap with succeeding segment. */
+	if (next && FRAG6_CB(next)->offset < end)
+		goto discard_fq;
 
 	FRAG6_CB(skb)->offset = offset;
 
 	/* Insert this fragment in the chain of fragments. */
 	skb->next = next;
@@ -434,10 +391,12 @@ static int ip6_frag_queue(struct frag_queue *fq, struct sk_buff *skb,
 	write_lock(&ip6_frags.lock);
 	list_move_tail(&fq->q.lru_list, &fq->q.net->lru_list);
 	write_unlock(&ip6_frags.lock);
 	return -1;
 
+discard_fq:
+	fq_kill(fq);
 err:
 	IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
 		      IPSTATS_MIB_REASMFAILS);
 	kfree_skb(skb);
 	return -1;

Vulnerable code: /* Memory Tracking Functions. */
static void frag_kfree_skb(struct netns_frags *nf, struct sk_buff *skb)
{
	atomic_sub(skb->truesize, &nf->mem);
	kfree_skb(skb);
}

	/* We found where to put this one.  Check for overlap with
	 * preceding fragment, and, if needed, align things so that
	 * any overlaps are eliminated.
	if (prev) {
		int i = (FRAG6_CB(prev)->offset + prev->len) - offset;
		if (i > 0) {
			offset += i;
			if (end <= offset)
				goto err;
			if (!pskb_pull(skb, i))
				goto err;
			if (skb->ip_summed != CHECKSUM_UNNECESSARY)
				skb->ip_summed = CHECKSUM_NONE;
		}
	}
	/* Look for overlap with succeeding segments.
	 * If we can merge fragments, do it.
	 */
	while (next && FRAG6_CB(next)->offset < end) {
		int i = end - FRAG6_CB(next)->offset; /* overlap is 'i' bytes */

		if (i < next->len) {
			/* Eat head of the next overlapped fragment
			 * and leave the loop. The next ones cannot overlap.
			 */
			if (!pskb_pull(next, i))
				goto err;
			FRAG6_CB(next)->offset += i;	/* next fragment */
			fq->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
				fq->q.fragments = next;

			fq->q.meat -= free_it->len;
			frag_kfree_skb(fq->q.net, free_it);
		}
	}
Fixed code: 	/* RFC5722, Section 4:
	 *                                  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, including those not yet received) MUST be silently
	 *   discarded.
	/* Check for overlap with preceding fragment. */
	if (prev &&
	    (FRAG6_CB(prev)->offset + prev->len) - offset > 0)
		goto discard_fq;
	/* Look for overlap with succeeding segment. */
	if (next && FRAG6_CB(next)->offset < end)
		goto discard_fq;
discard_fq:
	fq_kill(fq);