Re: [PATCH 2/3] io: Support shutdown of TLS channel

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Mar 27, 2020 at 12:29:39PM -0500, Eric Blake wrote:
> On 3/27/20 11:40 AM, Daniel P. Berrangé wrote:
> > On Fri, Mar 27, 2020 at 11:19:35AM -0500, Eric Blake wrote:
> > > Gnutls documents that while many apps simply yank out the underlying
> > > transport at the end of communication in the name of efficiency, this
> > > is indistinguishable from a malicious actor terminating the connection
> > > prematurely.  Since our channel I/O code already supports the notion of
> > > a graceful shutdown request, it is time to plumb that through to the
> > > TLS layer, and wait for TLS to give the all clear before then
> > > terminating traffic on the underlying channel.
> > > 
> > > Note that channel-tls now always advertises shutdown support,
> > > regardless of whether the underlying channel also has that support.
> > > 
> > > Signed-off-by: Eric Blake <eblake@redhat.com>
> > > ---
> > >   io/channel-tls.c | 27 ++++++++++++++++++++++++++-
> > >   1 file changed, 26 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/io/channel-tls.c b/io/channel-tls.c
> > > index 7ec8ceff2f01..f90905823e1d 100644
> > > --- a/io/channel-tls.c
> > > +++ b/io/channel-tls.c
> > > @@ -360,10 +360,35 @@ static int qio_channel_tls_shutdown(QIOChannel *ioc,
> > >                                       Error **errp)
> > >   {
> > >       QIOChannelTLS *tioc = QIO_CHANNEL_TLS(ioc);
> > > +    int ret = 0;
> > > 
> > >       tioc->shutdown |= how;
> > > 
> > > -    return qio_channel_shutdown(tioc->master, how, errp);
> > > +    do {
> > > +        switch (how) {
> > > +        case QIO_CHANNEL_SHUTDOWN_READ:
> > > +            /* No TLS counterpart */
> > > +            break;
> > > +        case QIO_CHANNEL_SHUTDOWN_WRITE:
> > > +            ret = qcrypto_tls_session_shutdown(tioc->session, QCRYPTO_SHUT_WR);
> > > +            break;
> > > +        case QIO_CHANNEL_SHUTDOWN_BOTH:
> > > +            ret = qcrypto_tls_session_shutdown(tioc->session,
> > > +                                               QCRYPTO_SHUT_RDWR);
> > > +            break;
> > > +        default:
> > > +            abort();
> > > +        }
> > > +    } while (ret == -EAGAIN);
> > 
> > I don't think it is acceptable to do this loop here. The gnutls_bye()
> > function triggers several I/O operations which could block. Looping
> > like this means we busy-wait, blocking this thread for as long as I/O
> > is blocking on the socket.
> 
> Hmm, good point.  Should we give qio_channel_tls_shutdown a bool parameter
> that says whether it should wait (good for use when we are being run in a
> coroutine and can deal with the additional I/O) or just fail with -EAGAIN
> (which the caller can ignore if it is not worried)?

A bool would not be sufficient, because the caller would need to know
which direction to wait for I/O on.

Looking at the code it does a flush of outstanding data, then sends
some bytes, and then receives some bytes. The write will probably
work most of the time, but the receive is almost always going to
return -EAGAIN. So I don't think that failing with EGAIN is going
to be much of a benefit.

> > If we must call gnutls_bye(), then it needs to be done in a way that
> > can integrate with the main loop so it poll()'s / unblocks the current
> > coroutine/thread.  This makes the whole thing significantly more
> > complex to deal with, especially if the shutdown is being done in
> > cleanup paths which ordinarily are expected to execute without
> > blocking on I/O.  This is the big reason why i never made any attempt
> > to use gnutls_bye().
> 
> We _are_ using gnutls_bye(GNUTLS_SHUT_RDWR) on the close() path (which is

Are you sure ?  AFAIK there is no existing usage of gnutls_bye() at all
in QEMU.

> indeed a cleanup path where not blocking is worthwhile) even without this
> patch, but the question is whether using gnutls_bye(GNUTLS_SHUT_WR) in the
> normal data path, where we are already using coroutines to manage callbacks,
> can benefit the remote endpoint, giving them a chance to see clean shutdown
> instead of abrupt termination.

I'm not convinced the clean shutdown at the TLS level does anything important
given that we already have done a clean shutdown at the NBD level.


Regards,
Daniel
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