net: ip6_gre: fix possible use-after-free in ip6erspan_rcv

erspan_v6 tunnels run __iptunnel_pull_header on received skbs to remove
erspan header. This can determine a possible use-after-free accessing
pkt_md pointer in ip6erspan_rcv since the packet will be 'uncloned'
running pskb_expand_head if it is a cloned gso skb (e.g if the packet has
been sent though a veth device). Fix it resetting pkt_md pointer after
__iptunnel_pull_header

Fixes: c295d817e49d ("net: erspan: refactor existing erspan code")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index b32c95f0212809006455cb79768f96bd1c516994..655e46b227f9eb99e43369ffb96a411bd662eadb 100644 (file)
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -525,10 +525,10 @@ static int ip6gre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi)
 }
 
 static int ip6erspan_rcv(struct sk_buff *skb,
-                        struct tnl_ptk_info *tpi)
+                        struct tnl_ptk_info *tpi,
+                        int gre_hdr_len)
 {
        struct erspan_base_hdr *ershdr;
-       struct erspan_metadata *pkt_md;
        const struct ipv6hdr *ipv6h;
        struct erspan_md2 *md2;
        struct ip6_tnl *tunnel;
@@ -547,18 +547,16 @@ static int ip6erspan_rcv(struct sk_buff *skb,
                if (unlikely(!pskb_may_pull(skb, len)))
                        return PACKET_REJECT;
 
-               ershdr = (struct erspan_base_hdr *)skb->data;
-               pkt_md = (struct erspan_metadata *)(ershdr + 1);
-
                if (__iptunnel_pull_header(skb, len,
                                           htons(ETH_P_TEB),
                                           false, false) < 0)
                        return PACKET_REJECT;
 
                if (tunnel->parms.collect_md) {
+                       struct erspan_metadata *pkt_md, *md;
                        struct metadata_dst *tun_dst;
                        struct ip_tunnel_info *info;
-                       struct erspan_metadata *md;
+                       unsigned char *gh;
                        __be64 tun_id;
                        __be16 flags;
 
@@ -571,6 +569,14 @@ static int ip6erspan_rcv(struct sk_buff *skb,
                        if (!tun_dst)
                                return PACKET_REJECT;
 
+                       /* skb can be uncloned in __iptunnel_pull_header, so
+                        * old pkt_md is no longer valid and we need to reset
+                        * it
+                        */
+                       gh = skb_network_header(skb) +
+                            skb_network_header_len(skb);
+                       pkt_md = (struct erspan_metadata *)(gh + gre_hdr_len +
+                                                           sizeof(*ershdr));
                        info = &tun_dst->u.tun_info;
                        md = ip_tunnel_info_opts(info);
                        md->version = ver;
@@ -607,7 +613,7 @@ static int gre_rcv(struct sk_buff *skb)
 
        if (unlikely(tpi.proto == htons(ETH_P_ERSPAN) ||
                     tpi.proto == htons(ETH_P_ERSPAN2))) {
-               if (ip6erspan_rcv(skb, &tpi) == PACKET_RCVD)
+               if (ip6erspan_rcv(skb, &tpi, hdr_len) == PACKET_RCVD)
                        return 0;
                goto out;
        }