On Mon, Jul 21, 2025 at 11:56:09AM -0300, Fabiano Rosas wrote: > Daniel P. Berrangé <berra...@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/de...@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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
