Re: [PATCH v4 08/33] migration/multifd: Allow premature EOF on TLS incoming channels

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

On Mon, Feb 03, 2025 at 01:20:01PM -0500, Peter Xu wrote:
> On Thu, Jan 30, 2025 at 11:08:29AM +0100, Maciej S. Szmigiero wrote:
> > From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> > 
> > Multifd send channels are terminated by calling
> > qio_channel_shutdown(QIO_CHANNEL_SHUTDOWN_BOTH) in
> > multifd_send_terminate_threads(), which in the TLS case essentially
> > calls shutdown(SHUT_RDWR) on the underlying raw socket.
> > 
> > Unfortunately, this does not terminate the TLS session properly and
> > the receive side sees this as a GNUTLS_E_PREMATURE_TERMINATION error.
> > 
> > The only reason why this wasn't causing migration failures is because
> > the current migration code apparently does not check for migration
> > error being set after the end of the multifd receive process.
> > 
> > However, this will change soon so the multifd receive code has to be
> > prepared to not return an error on such premature TLS session EOF.
> > Use the newly introduced QIOChannelTLS method for that.
> > 
> > It's worth noting that even if the sender were to be changed to terminate
> > the TLS connection properly the receive side still needs to remain
> > compatible with older QEMU bit stream which does not do this.
> 
> If this is an existing bug, we could add a Fixes.
> 
> Two pure questions..
> 
>   - What is the correct way to terminate the TLS session without this flag?
> 
>   - Why this is only needed by multifd sessions?

Graceful TLS termination (via gnutls_bye()) should only be important to
security if the QEMU protocol in question does not know how much data it
is expecting to recieve. ie it cannot otherwise distinguish between an
expected EOF, and a premature EOF triggered by an attacker.

If the migration protocol has sufficient info to know when a chanel is
expected to see EOF, then we should stop trying to read from the TLS
channel before seeing the underlying EOF.

Ignoring GNUTLS_E_PREMATURE_TERMINATION would be valid if we know that
migration will still fail corretly in the case of a malicious attack
causing premature termination.

If there's a risk that migration may succeed, but with incomplete data,
then we would need the full gnutls_bye dance.

With regards,
Daniel
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