netdev - Re: [PATCH v2] tcp: Initial support for RFC5925 auth option

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Message-ID: <9bcd27f0-e14e-ab89-88a4-f6cf6b4323b4@gmail.com>
Date:   Thu, 4 Nov 2021 00:22:49 +0200
From:   Leonard Crestez <cdleonard@...il.com>
To:     David Ahern <dsahern@...il.com>, David Ahern <dsahern@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        Francesco Ruggeri <fruggeri@...sta.com>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        Christoph Paasch <cpaasch@...le.com>,
        Ivan Delalande <colona@...sta.com>,
        Priyaranjan Jha <priyarjha@...gle.com>, netdev@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tcp: Initial support for RFC5925 auth option

On 11/3/21 5:18 AM, David Ahern wrote:
> On 11/1/21 10:34 AM, Leonard Crestez wrote:
>> This is similar to TCP MD5 in functionality but it's sufficiently
>> different that wire formats are incompatible. Compared to TCP-MD5 more
>> algorithms are supported and multiple keys can be used on the same
>> connection but there is still no negotiation mechanism.
>>
>> Expected use-case is protecting long-duration BGP/LDP connections
>> between routers using pre-shared keys. The goal of this series is to
>> allow routers using the linux TCP stack to interoperate with vendors
>> such as Cisco and Juniper.
>>
>> Both algorithms described in RFC5926 are implemented but the code is not
>> very easily extensible beyond that. In particular there are several code
>> paths making stack allocations based on RFC5926 maximum, those would
>> have to be increased.
>>
>> This version implements SNE and l3mdev awareness and adds more tests.
>> Here are some known flaws and limitations:
>>
>> * Interaction with TCP-MD5 not tested in all corners
>> * Interaction with FASTOPEN not tested and unlikely to work because
>> sequence number assumptions for syn/ack.
>> * Not clear if crypto_shash_setkey might sleep. If some implementation
>> do that then maybe they could be excluded through alloc flags.
>> * Traffic key is not cached (reducing performance)
>> * User is responsible for ensuring keys do not overlap.
>> * There is no useful way to list keys, making userspace debug difficult.
>> * There is no prefixlen support equivalent to md5. This is used in
>> some complex FRR configs.
>>
>> Test suite was added to tools/selftests/tcp_authopt. Tests are written
>> in python using pytest and scapy and check the API in some detail and
>> validate packet captures. Python code is already used in linux and in
>> kselftests but virtualenvs not very much, this particular test suite
>> uses `pip` to create a private virtualenv and hide dependencies.
>>
>> This actually forms the bulk of the series by raw line-count. Since
>> there is a lot of code it was mostly split on "functional area" so most
>> files are only affected by a single code. A lot of those tests are
>> relevant to TCP-MD5 so perhaps it might help to split into a separate
>> series?
>>
>> Some testing support is included in nettest and fcnal-test.sh, similar
>> to the current level of tcp-md5 testing.
>>
>> SNE was tested by creating connections in a loop until a large SEQ is
>> randomly selected and then making it rollover. The "connect in a loop"
>> step ran into timewait overflow and connection failure on port reuse.
>> After spending some time on this issue and my conclusion is that AO
>> makes it impossible to kill remainders of old connections in a manner
>> similar to unsigned or md5sig, this is because signatures are dependent
>> on ISNs.  This means that if a timewait socket is closed improperly then
>> information required to RST the peer is lost.
>>
>> The fact that AO completely breaks all connection-less RSTs is
>> acknowledged in the RFC and the workaround of "respect timewait" seems
>> acceptable.
>>
>> Changes for frr (old): https://github.com/FRRouting/frr/pull/9442
>> That PR was made early for ABI feedback, it has many issues.
>>
> 
> overall looks ok to me. I did not wade through the protocol details.
> 
> I did see the comment about no prefixlen support in the tests. A lot of
> patches to absorb, perhaps I missed it. Does AuthOpt support for
> prefixes? If not, you should consider adding that as a quick follow on
> (within the same dev cycle). MD5 added prefix support for scalability;
> seems like AO should be concerned about the same.

I just skipped it because it's not required for core functionality.

It's very straight forward so I will add it to the next version.

--
Regards,
Leonard