On Mon, Jul 21, 2025 at 12:14:51PM -0300, Fabiano Rosas wrote: > Daniel P. Berrangé <berra...@redhat.com> writes: > > > 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. > > > > 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 -- |: 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 :|
