xfrm: Fix null pointer dereference when decoding sessions

On some codepaths the skb does not have a dst entry
when xfrm_decode_session() is called. So check for
a valid skb_dst() before dereferencing the device
interface index. We use 0 as the device index if
there is no valid skb_dst(), or at reverse decoding
we use skb_iif as device interface index.

Bug was introduced with git commit bafd4bd4dc
("xfrm: Decode sessions with output interface.").

Reported-by: Meelis Roos <mroos@linux.ee>
Tested-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index 4764ee4..e1a6393 100644 (file)
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -104,10 +104,14 @@ _decode_session4(struct sk_buff *skb, struct flowi *fl, int reverse)
        const struct iphdr *iph = ip_hdr(skb);
        u8 *xprth = skb_network_header(skb) + iph->ihl * 4;
        struct flowi4 *fl4 = &fl->u.ip4;
+       int oif = 0;
+
+       if (skb_dst(skb))
+               oif = skb_dst(skb)->dev->ifindex;
 
        memset(fl4, 0, sizeof(struct flowi4));
        fl4->flowi4_mark = skb->mark;
-       fl4->flowi4_oif = skb_dst(skb)->dev->ifindex;
+       fl4->flowi4_oif = reverse ? skb->skb_iif : oif;
 
        if (!ip_is_fragment(iph)) {
                switch (iph->protocol) {

diff --git a/net/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c
index dd503a3..5f8e128 100644 (file)
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c
@@ -135,10 +135,14 @@ _decode_session6(struct sk_buff *skb, struct flowi *fl, int reverse)
        struct ipv6_opt_hdr *exthdr;
        const unsigned char *nh = skb_network_header(skb);
        u8 nexthdr = nh[IP6CB(skb)->nhoff];
+       int oif = 0;
+
+       if (skb_dst(skb))
+               oif = skb_dst(skb)->dev->ifindex;
 
        memset(fl6, 0, sizeof(struct flowi6));
        fl6->flowi6_mark = skb->mark;
-       fl6->flowi6_oif = skb_dst(skb)->dev->ifindex;
+       fl6->flowi6_oif = reverse ? skb->skb_iif : oif;
 
        fl6->daddr = reverse ? hdr->saddr : hdr->daddr;
        fl6->saddr = reverse ? hdr->daddr : hdr->saddr;