[SRU][Trusty][Zesty][PATCH v2 1/2] ipv6: avoid overflow of offset in ip6_find_1stfragopt

[Kleber Sacilotto de Souza]

From: Sabrina Dubroca <sd at queasysnail.net>

In some cases, offset can overflow and can cause an infinite loop in
ip6_find_1stfragopt(). Make it unsigned int to prevent the overflow, and
cap it at IPV6_MAXPLEN, since packets larger than that should be invalid.

This problem has been here since before the beginning of git history.

Signed-off-by: Sabrina Dubroca <sd at queasysnail.net>
Acked-by: Hannes Frederic Sowa <hannes at stressinduktion.org>
Signed-off-by: David S. Miller <davem at davemloft.net>

CVE-2017-7542
(cherry picked from commit 6399f1fae4ec29fab5ec76070435555e256ca3a6)
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
---
 net/ipv6/output_core.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/output_core.c b/net/ipv6/output_core.c
index 3764254f4523..cb80a45cd2d6 100644
--- a/net/ipv6/output_core.c
+++ b/net/ipv6/output_core.c
@@ -43,7 +43,7 @@ EXPORT_SYMBOL_GPL(ipv6_proxy_select_ident);
 
 int ip6_find_1stfragopt(struct sk_buff *skb, u8 **nexthdr)
 {
-	u16 offset = sizeof(struct ipv6hdr);
+	unsigned int offset = sizeof(struct ipv6hdr);
 	unsigned int packet_len = skb_tail_pointer(skb) -
 		skb_network_header(skb);
 	int found_rhdr = 0;
@@ -51,6 +51,7 @@ int ip6_find_1stfragopt(struct sk_buff *skb, u8 **nexthdr)
 
 	while (offset <= packet_len) {
 		struct ipv6_opt_hdr *exthdr;
+		unsigned int len;
 
 		switch (**nexthdr) {
 
@@ -76,7 +77,10 @@ int ip6_find_1stfragopt(struct sk_buff *skb, u8 **nexthdr)
 
 		exthdr = (struct ipv6_opt_hdr *)(skb_network_header(skb) +
 						 offset);
-		offset += ipv6_optlen(exthdr);
+		len = ipv6_optlen(exthdr);
+		if (len + offset >= IPV6_MAXPLEN)
+			return -EINVAL;
+		offset += len;
 		*nexthdr = &exthdr->nexthdr;
 	}
 
-- 
2.14.1