Re: [PATCH v6 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if an HMAC key is loaded

Mimi Zohar
2021-05-11T19:53:48Z

On Tue, 2021-05-11 at 14:12 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > Sent: Tuesday, May 11, 2021 3:42 PM
> > On Wed, 2021-05-05 at 13:29 +0200, Roberto Sassu wrote:
> > > EVM_ALLOW_METADATA_WRITES is an EVM initialization flag that can be
> > set to
> > > temporarily disable metadata verification until all xattrs/attrs necessary
> > > to verify an EVM portable signature are copied to the file. This flag is
> > > cleared when EVM is initialized with an HMAC key, to avoid that the HMAC is
> > > calculated on unverified xattrs/attrs.
> > >
> > > Currently EVM unnecessarily denies setting this flag if EVM is initialized
> > > with a public key, which is not a concern as it cannot be used to trust
> > > xattrs/attrs updates. This patch removes this limitation.
> > >
> > > Cc: stable@vger.kernel.org # 4.16.x
> > > Fixes: ae1ba1676b88e ("EVM: Allow userland to permit modification of EVM-
> > protected metadata")
> > > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> >
> > Once the comments below are addressed,
> >
> > Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
> >
> > > ---
> > > Documentation/ABI/testing/evm | 5 +++--
> > > security/integrity/evm/evm_secfs.c | 5 ++---
> > > 2 files changed, 5 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/Documentation/ABI/testing/evm
> > b/Documentation/ABI/testing/evm
> > > index 3c477ba48a31..eb6d70fd6fa2 100644
> > > --- a/Documentation/ABI/testing/evm
> > > +++ b/Documentation/ABI/testing/evm
> > > @@ -49,8 +49,9 @@ Description:
> > > modification of EVM-protected metadata and
> > > disable all further modification of policy
> > >
> > > - Note that once a key has been loaded, it will no longer be
> > > - possible to enable metadata modification.
> > > + Note that once an HMAC key has been loaded, it will no longer
> > > + be possible to enable metadata modification and, if it is
> > > + already enabled, it will be disabled.
> >
> > It's worth mentioning that echo'ing a new value is additive. Once EVM
> > metadata modification is enabled, the only way of disabling it is by
> > enabling an HMAC key. It's also worth mentioning that metadata writes
> > are only permitted once further changes to the EVM policy are disabled.
>
> If I'm not wrong, it is not required to set EVM_SETUP_COMPLETE to allow
> metadata writes.

Agreed, EVM_SETUP_COMPLETE is not needed to allow metadata writes.
Once EVM_ALLOW_METADATA_WRITES is enabled, however, there is no way of
unsetting it without loading the HMAC key.

> I think the original idea was to boot a system in a way
> that portable signatures can be written, and then to enable enforcement.

Nothing special is needed to write portable signatures. Based on the
documentation, I think the original intention supports three modes:
- only enable HMAC validation (1)
- enable both HMAC and digital signature validation (3)
- only enable digital signature validation and allow modification of
EVM-protected metadata (6)

The third example is enabled using "0x80000006", which also prevents
enabling HMAC verification. Leaving out the example of enabling just
digital signature validation without modification of EVM protected
metadata seems to have been intentional.

thanks,

Mimi