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SubjectRe: [OOPS][3.12] BUG: unable to handle kernel NULL pointer dereference at 0000000c
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From: Bjorn Helgaas <bhelgaas@google.com>
Date: Fri, 15 Nov 2013 15:29:53 -0700

> [+cc David, Eric, Alex, netdev]
>
> Alex reported a similar issue at
> http://marc.info/?l=linux-netdev&m=138355719901790&w=4

Fixed by:

commit 84502b5ef9849a9694673b15c31bd3ac693010ae
Author: Steffen Klassert <steffen.klassert@secunet.com>
Date: Wed Oct 30 11:16:28 2013 +0100

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
--- 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
--- 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;

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