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

[Fabiano Rosas <farosas@suse.de>]

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.