Unexpected EOF handling

[Dmitry Belyavsky]

Dear Kurt

On Fri, May 8, 2020 at 1:10 PM Kurt Roeckx <kurt at roeckx.be> wrote:

> On Thu, May 07, 2020 at 02:31:24PM +0200, Tomas Mraz wrote:
> > On Thu, 2020-05-07 at 12:47 +0100, Matt Caswell wrote:
> > >
> > > On 07/05/2020 12:22, Kurt Roeckx wrote:
> > > > So I think we need at least to agree on:
> > > > - Do we want an option that makes the unexpected EOF either a fatal
> > > >   error or a non-fatal error?
> > > > - Which error should we return?
> > >
> > > This is an excellent summary of the current situation.
> > >
> > > I am not keen on maintaining the SSL_ERROR_SYSCALL with 0 errno as a
> > > long term solution. It's a very confusing API for new applications to
> > > use. Effectively it means SSL_ERROR_SYSCALL is a fatal error - except
> > > when its not. SSL_ERROR_SYSCALL should mean fatal error.
> > >
> > > That said I also recognise that it is apparently commonplace to shut
> > > down a TLS connection without sending close_notify - despite what the
> > > standards may say about it (and TBH we can hardly claim the moral
> > > high
> > > ground here since s_server does exactly this - or at least it does in
> > > 1.1.1 and did until very recently in master).
> > >
> > > But we do have to consider usages beyond HTTPS. I have no idea if
> > > this
> > > occurs in other settings or not.
> > >
> > > Perhaps what we need is an option for "strict shutdown". With strict
> > > shutdown "off" we could treat EOF as if we had received a
> > > close_notify
> > > gracefully (and don't invalidate the session). Presumably existing
> > > code
> > > would be able to cope with that???
> >
> > Yes, existing code would be able to cope with that with one important
> > exception that I am going to talk about below.
> >
> > > With strict shutdown "on" we treat it as SSL_ERROR_SSL (as now in
> > > master).
> > >
> > > I'm not sure though what the default should be.
> >
> > In case we go with this solution, which would be acceptable I think, we
> > MUST NOT EVER make it the default because existing applications that
> > depend on the existing way how the unclean EOF condition is returned
> > might actually depend on it to detect the truncation attack.
>
> I agree that we should not return SSL_ERROR_ZERO_RETURN by default
> on an unexpected EOF.
>
> If the default behaviour should be to make it a non-fatal error,
> like the old behaviour is, I would really prefer a different
> error, one that's not SSL_ERROR_SYSCALL or SSL_ERROR_SSL.
>
> So I think the suggestion is to have this:
> - By default, SSL_ERROR_SSL is returned with
>   SSL_R_UNEXPECTED_EOF_WHILE_READING, the session will be
>   marked invalid.
> - With an option, SSL_ERROR_ZERO_RETURN is returned, the session
>   will stay valid.
>

If I remember correctly, session resumption is a way to significantly
reduce a server's workload.
So I think that by default (and maybe the only option) we should prefer the
old behaviour.

-- 
SY, Dmitry Belyavsky
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