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

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

On Mon, Jul 21, 2025 at 12:14:51PM -0300, Fabiano Rosas wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > 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.
> >
> 
> Makes sense. Thanks.
> 
> Will you take the series or should I?

I presume you've already got another migration pull request planned, so
feel free to add this


With regards,
Daniel
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