On 12/25/2023 8:00 AM, Guozihua (Scott) wrote:
After thinking about this some more, I realized that we didn't discuss
carrying the measurement list across kexec. How will the kexec'ed
kernel be able to differentiate between the original and new
measurement list? Neither the Kconfig nor making SHA1 backwards
compatible address this. How will attestation servers be able to
differentiate between the two logs?
Obviously the new measurement list with larger digests won't be
backwards compatible. Can we support carrying the original measurement
list across kexec to a new kernel?
As long we're making changes to the IMA measurement list, we should
mention that we could include multiple digests, but I don't think it is
necessary.
An example of the resulting ima log is the following:
10 sha256:64326[...]25313 ima-ng sha1:5fc9b[...]974e6 boot_aggregate
10 sha256:afd64[...]e3123 ima-ng sha1:5a493[...]f9566 /init
10 sha256:99329[...]a6353 ima-ng sha1:8c87d[...]3d8c7 /usr/bin/sh
10 sha256:a16ad[...]2ac0e ima-ng sha1:59d4b[...]330b0 /etc/ld.so.cache
The template DATA_FMT_DIGEST_WITH_ALGO field is a composite field
defined as <hash algo> + ':' + '\0' + digest. The above ascii display
looks like the template composite field, but isn't. It is two separate
fields. Perhaps it should look differently as well. Consider instead
of a string use the hash_algo value (include/uapi/linux/hash_info.h).
binary measurement log: <pcr> <hash_algo> <digest_len> <digest>
Hi Mimi and Enrico,
As we are extending all supported algorithms of a PCR bank, does
algorithm of template-hash still matters?
From a security and verification viewpoint, I think you are correct.
The template hash is redundant, and can always be calculated from the
template data. In that sense, the template hash can be removed.
On the other hand there is one benefit to the template hash. If there is
a bug in the software that creates the event log or the software that
consumes the event log, the template hash may help to determine which
event has a bug.
If the new event log has a template hash, I do believe that it also
needs a hash algorithm.
