[gnutls-help] 3.6.7 interoperability problems with earlier 3.6.x

[Lennert Buytenhek]

On Sat, Jun 08, 2019 at 11:29:39AM +0200, Andreas Metzler wrote:

> this is https://bugs.debian.org/929907 submitted and boiled down to a
> minimal (gnutls-cli) testcase by Dominik George. (The original issue was
> openldap breaking.)
> 
> gnutls-serv from 3.6.4 3.6.5 and 3.6.6 does not work correctly with
> gnutls-cli from 3.6.7.
> ------
> gnutls-serv --echo --x509keyfile /etc/ssl/private/ssl-cert-snakeoil.key --x509certfile /etc/ssl/certs/ssl-cert-snakeoil.pem
> ...
> pwgen 16383 | gnutls-cli --no-ca-verification --port 5556 localhost
> > From a size of 16383 bytes onwards, I get:
> 
> > |<1>| Received packet with illegal length: 16385
> > |<1>| Discarded message[1] due to invalid decryption
> > *** Fatal error: A TLS record packet with invalid length was received.
> > *** Server has terminated the connection abnormally.
> ------
> 
> gnutls-cli 3.6.8 works. Disabling TLS 1.3 or %NO_EXTENSIONS also works
> around this.
> 
> I have run git bisect to find more info. The issue was introduced in
> 3fc7d37dd81a1f415afffbf5f733c13296e74824
> ext/record_size_limit: account for content type octet in TLS 1.3
> 
> and was fixed for 3.6.8 by
> 2dc96e3b8d0e043bebf0815edaaa945f66ac0531
> ext/record_size_limit: distinguish sending and receiving limits
> 
> I am now wondering on what to do with this bug for the next Debian
> stable release ("buster").
> * We are unlikely to upgrade to 3.6.8 since buster is already frozen.
> * Both reverting 3fc7d37dd81a1f415afffbf5f733c13296e74824 and applying
>   2dc96e3b8d0e043bebf0815edaaa945f66ac0531 makes the issue
>   unreproducible. I don't like the former because it reverts a TLS 1.3 
>   bugfix. Straightforward applying of the latter would add
>   gnutls_record_set_max_recv_size(). (So I'd problably rename to
>   _gnutls_record_set_max_recv_size and patch to not export.)
> * I am not sure about the impact, whether we are unlucky we don't have a
>   failing testcase with openssl or whether it only shows up with Record
>   Size Limit Extension. However Ubuntu Cosmic Cuttlefish and Disco Dingo
>   ship 3.6.4 and 3.6.5 respectively, so we can't just ignore this, since
>   I really want Debian and Ubuntu release to work together. ;-)
> * Do the git bisect result make sense?

(I have no useful insights concerning your bug to add, other than to
remark that this sounds similar to an issue I ran into a while ago:

	https://lists.gnupg.org/pipermail/gnutls-help/2019-January/004472.html

Apologies in case this is off-topic.)