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? > With regards, > Daniel
