linux-kernel - Re: [PATCH 03/11] evm: Refuse EVM_ALLOW_METADATA

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Date:   Mon, 31 Aug 2020 17:31:26 -0400
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Roberto Sassu <roberto.sassu@...wei.com>,
        "mjg59@...gle.com" <mjg59@...gle.com>
Cc:     "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Silviu Vlasceanu <Silviu.Vlasceanu@...wei.com>
Subject: Re: [PATCH 03/11] evm: Refuse EVM_ALLOW_METADATA_WRITES only if the
 HMAC key is loaded

On Mon, 2020-08-31 at 08:24 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@...ux.ibm.com]
> > Sent: Friday, August 21, 2020 10:15 PM
> > Hi Roberto,
> > 
> > On Thu, 2020-06-18 at 18:01 +0200, Roberto Sassu wrote:
> > > Granting metadata write is safe if the HMAC key is not loaded, as it won't
> > > let an attacker obtain a valid HMAC from corrupted xattrs.
> > evm_write_key()
> > > however does not allow it if any key is loaded, including a public key,
> > > which should not be a problem.
> > >
> > 
> > Why is the existing hebavior a problem?  What is the problem being
> > solved?
> 
> Hi Mimi
> 
> currently it is not possible to set EVM_ALLOW_METADATA_WRITES when
> only a public key is loaded and the HMAC key is not. The patch removes
> this limitation.

Yes, I understand.  You're describing "what" the problem is, not "why"
this is a problem.  Support for loading EVM HMAC and x509 certificates
isn't new.  Please add a line or two prior to this paragraph providing
the context for why this is now a problem.

Is the problem related to previoulsy not beginning EVM verification
until after the EVM HMAC key was loaded?  Or perhaps EVM signatures
were not that common since they weren't portable.   Now, with portable
and immutable signatures loading x509 certificates is more common.

thanks,

Mimi