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

Roberto Sassu roberto.sassu at huawei.com
Fri May 21 07:07:14 UTC 2021

> From: Mimi Zohar [mailto:zohar at linux.ibm.com]
> Sent: Thursday, May 20, 2021 8:56 PM
> 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

Hi Mimi

could you please take the newer version of patch 5/12, which also adds
an exception for the INTEGRITY_UNKNOWN error (it occurs when xattrs
are not supported)?




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