ipip6 - integer underrun when handlince icmpv4 unreachable messages

Hello netdev,

the current code to translate icmpv4 "destination unreachable" packets to icmpv6 is later generating an integer underrun when calling icmpv6_send, by later calling skb_network_header_len and subtracting a bigger number from a lower one.

The issue is not visible for vanilla kernels and works correctly from a user perspective, but a kernel with the PAX patches and enabled "size overflow protection" will panic immediately when it's getting an icmpv4 destination unreachable packet back for an encapsulated ipv6 packet. (Remote tunnel endpoint not reachable.)

I think I've tracked the issue down and can show you the problem with the code... as I understand it as non-programmer greping the sources and googeling functions. I was even able to find a fix which passes the functionality test, but I'm unqualified to rate the correctness of it and so are reaching out to you for that.

What happens (output of printk's) with transport_header and network_header around "skb_reset_network_header" is described below the function.

Near the end of the mail there are two links to the gentoo bug tracker and pax forum, suggesting to put this forward to the lkml/netdev for review, also including more details on the panics.

So here the function "ipip6_err_gen_icmpv6_unreach" from net/ipv6/sit.c with some remarks and one new line which seems to fix the problem:

************************************************************************
static int ipip6_err_gen_icmpv6_unreach(struct sk_buff *skb)

{
        int ihl = ((const struct iphdr *)skb->data)->ihl*4;
        struct rt6_info *rt;
        struct sk_buff *skb2;

        if (!pskb_may_pull(skb, ihl + sizeof(struct ipv6hdr) + 8))
                return 1;

// we clone the ipv4 skb in skb2 to prepare the icmpv6 packet

        skb2 = skb_clone(skb, GFP_ATOMIC);

        if (!skb2)
                return 1;

// we clean up the cloned skb2

        skb_dst_drop(skb2);
        skb_pull(skb2, ihl);
// The network header is reset
        skb_reset_network_header(skb2);

//THE PROPOSED FIX: The following line is NOT in the current code

        skb_reset_transport_header(skb2);

Quoted 6 lines · Alexander Wetzel

Hello netdev,

the current code to translate icmpv4 "destination unreachable" packets to icmpv6 is later generating an integer underrun when calling icmpv6_send, by later calling skb_network_header_len and subtracting a bigger number from a lower one.

I'd guess the call to skb_network_header_len() in _decode_session6() is the problematic one, right?

Quoted 41 lines

The issue is not visible for vanilla kernels and works correctly from a user perspective, but a kernel with the PAX patches and enabled "size overflow protection" will panic immediately when it's getting an icmpv4 destination unreachable packet back for an encapsulated ipv6 packet. (Remote tunnel endpoint not reachable.)

I think I've tracked the issue down and can show you the problem with the code... as I understand it as non-programmer greping the sources and googeling functions. I was even able to find a fix which passes the functionality test, but I'm unqualified to rate the correctness of it and so are reaching out to you for that.

What happens (output of printk's) with transport_header and network_header around "skb_reset_network_header" is described below the function.

Near the end of the mail there are two links to the gentoo bug tracker and pax forum, suggesting to put this forward to the lkml/netdev for review, also including more details on the panics.

So here the function "ipip6_err_gen_icmpv6_unreach" from net/ipv6/sit.c with some remarks and one new line which seems to fix the problem:

************************************************************************
static int ipip6_err_gen_icmpv6_unreach(struct sk_buff *skb)

{
        int ihl = ((const struct iphdr *)skb->data)->ihl*4;
        struct rt6_info *rt;
        struct sk_buff *skb2;

        if (!pskb_may_pull(skb, ihl + sizeof(struct ipv6hdr) + 8))
                return 1;

// we clone the ipv4 skb in skb2 to prepare the icmpv6 packet

        skb2 = skb_clone(skb, GFP_ATOMIC);

        if (!skb2)
                return 1;

Quoted 8 lines

// we clean up the cloned skb2

        skb_dst_drop(skb2);
        skb_pull(skb2, ihl);
// The network header is reset
        skb_reset_network_header(skb2);

//THE PROPOSED FIX: The following line is NOT in the current code

        skb_reset_transport_header(skb2);

This will make your panic with PAX go away, but the offset to the transport header is still wrong. Also, it is not just the sit tunnel that has this problem. All ipv6 tunnels are affected.

I think the easiest is to fix it in _decode_session6(). Could you please try the patch below?

Subject: [PATCH] xfrm6: Fix transport header offset in _decode_session6.

skb->transport_header might not be valid when we do a reverse decode because the ipv6 tunnel error handlers don't update it to the inner transport header. This leads to a wrong offset calculation and to wrong layer 4 informations. We fix this by using the size of the ipv6 header as the first offset.

Signed-off-by Steffen Klassert <[email protected]>

---
 net/ipv6/xfrm6_policy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

net/ipv6/xfrm6_policy.c +1 -1

index ac49f84..576fc42 100644
@@ -130,8 +130,8 @@ _decode_session6(struct sk_buff *skb, struct flowi *fl, int reverse)
 {
 	struct flowi6 *fl6 = &fl->u.ip6;
 	int onlyproto = 0;
-	u16 offset = skb_network_header_len(skb);
 	const struct ipv6hdr *hdr = ipv6_hdr(skb);
+	u16 offset = sizeof(*hdr);
 	struct ipv6_opt_hdr *exthdr;
 	const unsigned char *nh = skb_network_header(skb);
 	u8 nexthdr = nh[IP6CB(skb)->nhoff];

Hello,

The patch works for me, thank you very much.

Rejecting the ipv4 tunel packets with icmp unreachable is producing the expected "Destination unreachable: Address unreachable" messages for the inside ipv6 client and everything works as expected. Without the call in sit.c to skb_reset_transport_header.

I'll continue to use the patch on my ipv6 tunnel router but I'm sure it works as intended.

Quoted 13 lines · Steffen Klassert

I think the easiest is to fix it in _decode_session6(). Could you please try the patch below?

Subject: [PATCH] xfrm6: Fix transport header offset in _decode_session6.

skb->transport_header might not be valid when we do a reverse decode because the ipv6 tunnel error handlers don't update it to the inner transport header. This leads to a wrong offset calculation and to wrong layer 4 informations. We fix this by using the size of the ipv6 header as the first offset.

Signed-off-by Steffen Klassert <[email protected]>