[WireGuard] Source address fib invalidation on IPv6

[David Ahern]

On 11/12/16 8:40 AM, Jason A. Donenfeld wrote:
> Hi again,
> 
> I've done some pretty in depth debugging now to determine exactly what
> the behavior of ipv6_stub->ipv6_dst_lookup is. First I'll start with
> ip_route_output_flow, which I believe to be well behaved, and then
> I'll show ipv6_stub->ipv6_dst_lookup, which seems ill-behaved:
> 
> Userspace:
>     ip addr add 192.168.1.2/24 dev eth0
> Kernelspace:
>     struct flowi4 fl = {
>        .saddr = 192.168.1.2,
>        .daddr = 192.168.1.99,
>     };
>     rt = ip_route_output_flow(sock_net(sock), &fl, sock);
>     // rt returns valid rt for routing to 192.168.1.99 from
> 192.168.1.2 using eth0
> Userspace:
>     ip addr add 192.168.1.3/24 dev eth0
>     ip addr del 192.168.1.2/24 dev eth0
> Kernelspace:
>     struct flowi4 fl = {
>        .saddr = 192.168.1.2,
>        .daddr = 192.168.1.99,
>     };
>     rt = ip_route_output_flow(sock_net(sock), &fl, sock);
>     // PTR_ERR(rt) == -EINVAL

I believe that is coming from __ip_route_output_key_hash(), line 2232 with __ip_dev_find not finding a device with that address. 

Not applicable for your use case, but __ip_dev_find does not have any checks on which L3 domain the device belongs to so the check does not handle VRF for example. I'll take a look at fixing this next week.

> 
> This seems correct behavior to me, since no interface has 192.168.1.2
> as a source address.
> 
> Now for the incorrect IPv6 behavior:
> 
> Userspace:
>     ip -6 addr add abcd::2/96 dev eth0
> Kernelspace:
>     struct flowi6 fl = {
>        .saddr = abcd::2,
>        .daddr = abcd::99,
>     };
>     ret = ipv6_stub->ipv6_dst_lookup(sock_net(sock), sock, &dst, &fl);
>     // ret is 0, and dst is a non-null dst routing to abcd::99 from
> abcd::2 using eth0
> Userspace:
>     ip -6 addr add abcd::3/96 dev eth0
>     ip -6 addr del abcd::2/96 dev eth0
> Kernelspace:
>     struct flowi6 fl = {
>        .saddr = abcd::2,
>        .daddr = abcd::99,
>     };
>     ret = ipv6_stub->ipv6_dst_lookup(sock_net(sock), sock, &dst, &fl);
>     // ret is 0, and dst is a non-null dst routing to abcd::99 from
> abcd::2 using eth0 **INCORRECT BEHAVIOR!**
> 
> This seems *INCORRECT* behavior to me, since no interface has abcd::2
> as a source address.

Gotcha. I don't see any checks that the saddr is valid similar to what IPv4 does.

I think the right place to add a check is in ip6_dst_lookup_tail():
    if (!ipv6_addr_any(&fl6->saddr)) {
        // saddr is valid for L3 domain
    }