Re: [PATCH 0/4] migration: workaround GNUTLS live migration crashes

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

On Mon, Jul 21, 2025 at 11:56:09AM -0300, Fabiano Rosas wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > TL:DR: GNUTLS is liable to crash QEMU when live migration is run
> > with TLS enabled and a return path channel is present, if approx
> > 64 GB of data is transferred. This is easily triggered in a 16 GB
> > VM with 4 CPUs, by running 'stress-ng --vm 4 --vm-bytes 80%' to
> > prevent convergance until 64 GB of RAM has been copied. Then
> > triggering post-copy switchover, or removing the stress workload
> > to allow completion, will crash it.
> >
> > The only live migration scenario that should avoid this danger
> > is multifd, since the high volume data transfers are handled in
> > dedicated TCP connections which are unidirectional. The main
> > bi-directionl TCP connection is only for co-ordination purposes
> >
> > This patch implements a workaround that will prevent future QEMU
> > versions from triggering the crash.
> >
> > The only way to avoid the crash with *existing* running QEMU
> > processes is to change the TLS cipher priority string to avoid
> > use of AES with TLS 1.3. This can be done with the 'priority'
> > field in the 'tls-creds-x509' object.eg
> >
> >   -object tls-creds-x509,id=tls0,priority=NORMAL:-AES-256-GCM:-AES-128-GCM:-AES-128-CCM
> >
> > which should force the use of CHACHA20-POLY1305 which does not
> > require TLS re-keying after 16 million sent records (64 GB of
> > migration data).
> >
> >   https://gitlab.com/qemu-project/qemu/-/issues/1937
> >
> > On RHEL/Fedora distros you can also use the system wide crypto
> > priorities to override this from the migration *target* host
> > by creating /etc/crypto-policies/local.d/gnutls-qemu.config
> > containing
> >
> >   QEMU=NONE:+ECDHE-RSA:+ECDHE-ECDSA:+RSA:+DHE-RSA:+GROUP-X25519:+GROUP-X448:+GROUP-SECP256R1:+GROUP-SECP384R1:+GROUP-SECP521R1:+GROUP-FF
> >
> > and running 'update-crypto-policies'. I recommend the QEMU
> > level 'tls-creds-x509' workaround though, which new libvirt
> > patches can soon do:
> >
> >   https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/thread/LX5KMIUFZSP5DPUXKJDFYBZI5TIE3E5N/
> >
> > Daniel P. Berrangé (4):
> >   crypto: implement workaround for GNUTLS thread safety problems
> >   io: add support for activating TLS thread safety workaround
> >   migration: activate TLS thread safety workaround
> >   crypto: add tracing & warning about GNUTLS countermeasures
> >
> >  crypto/tlssession.c           | 99 +++++++++++++++++++++++++++++++++--
> >  crypto/trace-events           |  2 +
> >  include/crypto/tlssession.h   | 14 +++++
> >  include/io/channel.h          |  1 +
> >  io/channel-tls.c              |  5 ++
> >  meson.build                   |  9 ++++
> >  meson_options.txt             |  2 +
> >  migration/tls.c               |  9 ++++
> >  scripts/meson-buildoptions.sh |  5 ++
> >  9 files changed, 143 insertions(+), 3 deletions(-)
> 
> Hi, thank you for getting to the bottom of this.
> 
> Do you think it would be too cumbersome to add a test for this
> somewhere? So we don't regress the workaround but also so the test tells
> us whether GNUTLS is fixed.

The reproducer scenario is very expensive. I'm doing it with a 16 GB RAM
guest, with 4 CPUs, running 'stress-ng' guest workload. With that, it
takes between 10-20 minutes before live migration gets GNUTLS into the
potentially broken state, and the failure is not 100% guaranteed at
that point.

With regards,
Daniel
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