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.
