Unexpected EOF handling

[Tomas Mraz]

On Fri, 2020-05-08 at 12:09 +0200, Kurt Roeckx 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.

+1

And my suggestion for the SSL_OP name is SSL_OP_IGNORE_UNEXPECTED_EOF.

Dmitry, I think this solution should be working well for nginx and
similar http related applications. They just need to use the
SSL_OP_IGNORE_UNEXPECTED_EOF and the peers that do not properly
terminate the TLS session will just appear as if they properly
terminated it.

-- 
Tomáš Mráz
No matter how far down the wrong road you've gone, turn back.
                                              Turkish proverb
[You'll know whether the road is wrong if you carefully listen to your
conscience.]