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

[Roberto Sassu <roberto.sassu@huawei.com>]

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

Right, I didn't consider it.

Would it make sense instead to introduce an alias like security.evm_immutable
so that this xattr can be copied?

> At least IMA file hashes should always be used in conjunction with
> EVM.  EVM xattrs should always require a security.ima xattr to bind

I proposed to enforce this restriction some time ago:

https://patchwork.kernel.org/patch/10979351/

Is it ok to enforce it globally?

> the file metadata to the file data.  The IMA and EVM policies really
> need to be in sync.

It would be nice, but at the moment EVM considers also files that are
not selected by the IMA policy. An example of why this is a problem is
the audit service that fails to start when it tries to adjust the permissions
of the log files. Those files don't have security.evm because they are
not appraised by IMA, but EVM denies the operation.

Roberto

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