[RFC][PATCH 1/3] evm: Move hooks outside LSM infrastructure

Mimi Zohar zohar at linux.ibm.com
Thu May 7 15:17:08 UTC 2020

On Thu, 2020-05-07 at 07:53 +0000, Roberto Sassu wrote:
> > -----Original Message-----
> > From: Mimi Zohar [mailto:zohar at linux.ibm.com]
> > Sent: Wednesday, May 6, 2020 11:10 PM
> > To: Roberto Sassu <roberto.sassu at huawei.com>; david.safford at gmail.com;
> > viro at zeniv.linux.org.uk; jmorris at namei.org; John Johansen
> > <john.johansen at canonical.com>
> > Cc: linux-fsdevel at vger.kernel.org; linux-integrity at vger.kernel.org; linux-
> > security-module at vger.kernel.org; linux-kernel at vger.kernel.org; Silviu
> > Vlasceanu <Silviu.Vlasceanu at huawei.com>
> > Subject: Re: [RFC][PATCH 1/3] evm: Move hooks outside LSM infrastructure

Roberto, please fix your mailer or at least manually remove this sort
of info from the email.

> > 
> > On Wed, 2020-05-06 at 15:44 -0400, Mimi Zohar wrote:
> > > Since copying the EVM HMAC or original signature isn't applicable, I
> > > would prefer exploring an EVM portable and immutable signature only
> > > solution.
> > 
> > To prevent copying the EVM xattr, we added "security.evm" to
> > /etc/xattr.conf.  To support copying just the EVM portable and
> > immutable signatures will require a different solution.
> This patch set removes the need for ignoring security.evm. It can be always
> copied, even if it is an HMAC. EVM will update it only when verification in
> the pre hook is successful. Combined with the ability of protecting a subset
> of files without introducing an EVM policy, these advantages seem to
> outweigh the effort necessary to make the switch.

As the EVM file HMAC and original signature contain inode specific
information (eg. i_version, i_generation), these xattrs cannot ever be
copied.  The proposed change is in order to support just the new EVM

At least IMA file hashes should always be used in conjunction with
EVM.  EVM xattrs should always require a security.ima xattr to bind
the file metadata to the file data.  The IMA and EVM policies really
need to be in sync.


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