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

[Fabiano Rosas <farosas@suse.de>]

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?

> With regards,
> Daniel