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

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On 14.11.2016 18:17, David Ahern wrote:
> On 11/14/16 10:04 AM, Hannes Frederic Sowa wrote:
>> On 14.11.2016 17:55, David Ahern wrote:
>>> On 11/14/16 9:44 AM, Hannes Frederic Sowa wrote:
>>>> 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@zx2c4.com>
>>>>> Cc: David Ahern <dsa@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.
>>>
>>> This check -- like the ipv4 variant -- only verifies the saddr is locally assigned. If the address moves interfaces it should be fine.
>>
>> But in this case we should actually bail out, no?
>>
>> Let's say, user assumes we are on ifindex eth0 with LL address from
>> eth0. Suddenly the LL address from eth0 is moved to eth1, we can't
>> accept this source address anymore and need to return -EINVAL, too.
> 
> so you mean if rt6_need_strict(&fl6->saddr) then the dev needs to be considered.

Exactly, like we do in the user space facing APIs.

>>>> 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?
>>>
>>> I checked the userspace API this morning. bind and cmsg for example check that the address is valid with calls to ipv6_chk_addr.
>>
>> Hmm, so it fixes no real bug.
>>
>> Because of translations of flowi6_oif we actually can't do a correct
>> check of source address for cases like the one I outlined above? Hmm,
>> maybe we should simply depend on user space checks.
> 
> I believe Jason's case is forwarding path and the ipv6_stub->ipv6_dst_lookup API.

It is not a kernel API, because we don't support something like that for
external kernel modules. We basically exported ipv6_dst_lookup to allow
some IPv4 code to do ipv6 stunts when the IPv6 module is loaded. ;)

Bye,
Hannes