[WireGuard] [PATCH v3] ip6_output: ensure flow saddr actually belongs to device

[Hannes Frederic Sowa]

On Mon, Nov 14, 2016, at 00:28, Jason A. Donenfeld wrote:
> This puts the IPv6 routing functions in parity with the IPv4 routing
> functions. Namely, we now check in v6 that if a flowi6 requests an
> saddr, the returned dst actually corresponds to a net device that has
> that saddr. This mirrors the v4 logic with __ip_dev_find in
> __ip_route_output_key_hash. In the event that the returned dst is not
> for a dst with a dev that has the saddr, we return -EINVAL, just like
> v4; this makes it easy to use the same error handlers for both cases.
> 
> Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
> Cc: David Ahern <dsa at cumulusnetworks.com>
> ---
> Changes from v2:
>     It turns out ipv6_chk_addr already has the device enumeration
>     logic that we need by simply passing NULL.
> 
>  net/ipv6/ip6_output.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 6001e78..b3b5cb6 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -926,6 +926,10 @@ static int ip6_dst_lookup_tail(struct net *net,
> const struct sock *sk,
>  	int err;
>  	int flags = 0;
>  
> +       if (!ipv6_addr_any(&fl6->saddr) &&
> +           !ipv6_chk_addr(net, &fl6->saddr, NULL, 1))
> +               return -EINVAL;

Hmm, this check is too permissive, no?

E.g. what happens if you move a link local address from one interface to
another? In this case this code would still allow the saddr to be used.

I just also quickly read up on the history (sorry was travelling last
week) and wonder if you ever saw a user space facing bug or if this is
basically some difference you saw while writing out of tree code?

Thanks,
Hannes