netdev - Re: [PATCH] tcp: md5: Fix overlap between vrf and non-vrf keys

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Date:   Fri, 8 Oct 2021 18:51:56 +0300
From:   Leonard Crestez <cdleonard@...il.com>
To:     David Ahern <dsahern@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Ahern <dsahern@...nel.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
        Yonghong Song <yhs@...com>,
        Alexander Duyck <alexanderduyck@...com>,
        Florian Westphal <fw@...len.de>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tcp: md5: Fix overlap between vrf and non-vrf keys

On 07.10.2021 21:27, David Ahern wrote:
> On 10/7/21 12:41 AM, Leonard Crestez wrote:
>> On 07.10.2021 04:14, David Ahern wrote:
>>> On 10/6/21 11:48 AM, Leonard Crestez wrote:
>>>> @@ -1103,11 +1116,11 @@ static struct tcp_md5sig_key
>>>> *tcp_md5_do_lookup_exact(const struct sock *sk,
>>>>    #endif
>>>>        hlist_for_each_entry_rcu(key, &md5sig->head, node,
>>>>                     lockdep_sock_is_held(sk)) {
>>>>            if (key->family != family)
>>>>                continue;
>>>> -        if (key->l3index && key->l3index != l3index)
>>>> +        if (key->l3index != l3index)
>>>
>>> That seems like the bug fix there. The L3 reference needs to match for
>>> new key and existing key. I think the same change is needed in
>>> __tcp_md5_do_lookup.
>>
>> Current behavior is that keys added without tcpm_ifindex will match
>> connections both inside and outside VRFs. Changing this might break real
>> applications, is it really OK to claim that this behavior was a bug all
>> along?
> 
> no.
> 
> It's been a few years. I need to refresh on the logic and that is not
> going to happen before this weekend.

It seems that always doing a strict key->l3index != l3index condition 
inside of __tcp_md5_do_lookup breaks the usecase of binding one listener 
to each VRF and not specifying the ifindex for each key.

This is a very valid usecase, maybe the most common way to use md5 with vrf.

Ways to fix this:
* Make this comparison only take effect if TCP_MD5SIG_FLAG_IFINDEX is set.
* Make this comparison only take effect if tcp_l3mdev_accept=1
* Add a new flag?

Right now passing TCP_MD5SIG_FLAG_IFINDEX and ifindex == 0 results in an 
error but maybe it should be accepted to mean "key applies only for 
default VRF".

--
Regards,
Leonard