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On the host OS, various aspects of TLS operation are configurable.
In particular it is possible for the sysadmin to control the TLS
cipher/protocol algorithms that applications are permitted to use.
* Any given crypto library has a built-in default priority list
defined by the distro maintainer of the library package (or by
upstream).
* The "crypto-policies" RPM (or equivalent host OS package)
provides a config file such as "/etc/crypto-policies/config",
where the sysadmin can set a high level (library-independent)
policy.
The "update-crypto-policies --set" command (or equivalent) is
used to translate the global policy to individual library
representations, producing files such as
"/etc/crypto-policies/back-ends/*.config". The generated files,
if present, are loaded by the various crypto libraries to
override their own built-in defaults.
For example, the GNUTLS library may read
"/etc/crypto-policies/back-ends/gnutls.config".
* A management application (or the QEMU user) may overide the
system-wide crypto-policies config via their own config, if
they need to diverge from the former.
Thus the priority order is "QEMU user config" > "crypto-policies
system config" > "library built-in config".
Introduce the "tls-cipher-suites" object for exposing the ordered
list of permitted TLS cipher suites from the host side to the
guest firmware, via fw_cfg. The list is represented as an array
of bytes.
The priority at which the host-side policy is retrieved is given
by the "priority" property of the new object type. For example,
"priority=@SYSTEM" may be used to refer to
"/etc/crypto-policies/back-ends/gnutls.config" (given that QEMU
uses GNUTLS).
The firmware uses the IANA_TLS_CIPHER array for configuring
guest-side TLS, for example in UEFI HTTPS Boot.
[Description from Daniel P. Berrangé, edited by Laszlo Ersek.]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200623172726.21040-2-philmd@redhat.com>
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