dccp: Fix out of bounds access in DCCP error handler

There was a previous attempt to fix an out-of-bounds access in the DCCP
error handlers, but that fix assumed that the error handlers only want
to access the first 8 bytes of the DCCP header. Actually, they also look
at the DCCP sequence number, which is stored beyond 8 bytes, so an
explicit pskb_may_pull() is required.

Fixes: 6706a97f ("dccp: fix out of bound access in dccp_v4_err()")
Fixes: 1aa9d1a0 ("ipv6: dccp: fix out of bound access in dccp_v6_err()")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ipv4.c

@@ -255,12 +255,17 @@ static int dccp_v4_err(struct sk_buff *skb, u32 info)
 	int err;
 	struct net *net = dev_net(skb->dev);
 
-	/* Only need dccph_dport & dccph_sport which are the first
-	 * 4 bytes in dccp header.
+	/* For the first __dccp_basic_hdr_len() check, we only need dh->dccph_x,
+	 * which is in byte 7 of the dccp header.
 	 * Our caller (icmp_socket_deliver()) already pulled 8 bytes for us.
+	 *
+	 * Later on, we want to access the sequence number fields, which are
+	 * beyond 8 bytes, so we have to pskb_may_pull() ourselves.
 	 */
-	BUILD_BUG_ON(offsetofend(struct dccp_hdr, dccph_sport) > 8);
-	BUILD_BUG_ON(offsetofend(struct dccp_hdr, dccph_dport) > 8);
+	dh = (struct dccp_hdr *)(skb->data + offset);
+	if (!pskb_may_pull(skb, offset + __dccp_basic_hdr_len(dh)))
+		return -EINVAL;
+	iph = (struct iphdr *)skb->data;
 	dh = (struct dccp_hdr *)(skb->data + offset);
 
 	sk = __inet_lookup_established(net, &dccp_hashinfo,

net/dccp/ipv6.c

@@ -74,7 +74,7 @@ static inline __u64 dccp_v6_init_sequence(struct sk_buff *skb)
 static int dccp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 			u8 type, u8 code, int offset, __be32 info)
 {
-	const struct ipv6hdr *hdr = (const struct ipv6hdr *)skb->data;
+	const struct ipv6hdr *hdr;
 	const struct dccp_hdr *dh;
 	struct dccp_sock *dp;
 	struct ipv6_pinfo *np;
@@ -83,12 +83,17 @@ static int dccp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	__u64 seq;
 	struct net *net = dev_net(skb->dev);
 
-	/* Only need dccph_dport & dccph_sport which are the first
-	 * 4 bytes in dccp header.
+	/* For the first __dccp_basic_hdr_len() check, we only need dh->dccph_x,
+	 * which is in byte 7 of the dccp header.
 	 * Our caller (icmpv6_notify()) already pulled 8 bytes for us.
+	 *
+	 * Later on, we want to access the sequence number fields, which are
+	 * beyond 8 bytes, so we have to pskb_may_pull() ourselves.
 	 */
-	BUILD_BUG_ON(offsetofend(struct dccp_hdr, dccph_sport) > 8);
-	BUILD_BUG_ON(offsetofend(struct dccp_hdr, dccph_dport) > 8);
+	dh = (struct dccp_hdr *)(skb->data + offset);
+	if (!pskb_may_pull(skb, offset + __dccp_basic_hdr_len(dh)))
+		return -EINVAL;
+	hdr = (const struct ipv6hdr *)skb->data;
 	dh = (struct dccp_hdr *)(skb->data + offset);
 
 	sk = __inet6_lookup_established(net, &dccp_hashinfo,