Re: [PATCH v7 00/12] evm: Improve usability of portable signatures

[Mimi Zohar]

On Fri, 2021-05-14 at 17:27 +0200, Roberto Sassu wrote:
> EVM portable signatures are particularly suitable for the protection of
> metadata of immutable files where metadata is signed by a software vendor.
> They can be used for example in conjunction with an IMA policy that
> appraises only executed and memory mapped files.
> 
> However, until now portable signatures can be properly installed only if
> the EVM_ALLOW_METADATA_WRITES initialization flag is also set, which
> disables metadata verification until an HMAC key is loaded. This will cause
> metadata writes to be allowed even in the situations where they shouldn't
> (metadata protected by a portable signature is immutable).
> 
> The main reason why setting the flag is necessary is that the operations
> necessary to install portable signatures and protected metadata would be
> otherwise denied, despite being legitimate, due to the fact that the
> decision logic has to avoid an unsafe recalculation of the HMAC that would
> make the unsuccessfully verified metadata valid. However, the decision
> logic is too coarse, and does not fully take into account all the possible
> situations where metadata operations could be allowed.
> 
> For example, if the HMAC key is not loaded and it cannot be loaded in the
> future due the EVM_SETUP_COMPLETE flag being set, it wouldn't be a problem
> to allow metadata operations, as they wouldn't result in an HMAC being
> recalculated.
> 
> This patch set extends the decision logic and adds the necessary exceptions
> to use portable signatures without turning off metadata verification and
> deprecates the EVM_ALLOW_METADATA_WRITES flag.

Thanks, Roberto.

Applied to: git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-
integrity.git 
next-integrity-testing

Mimi