RE: [PATCH v5 09/12] evm: Allow setxattr() and setattr() for unmodified metadata

From: Roberto Sassu <roberto.sassu@huawei.com>
To: Mimi Zohar <zohar@linux.ibm.com>, "mjg59@google.com" <mjg59@google.com>
Cc: "linux-integrity@vger.kernel.org"
	<linux-integrity@vger.kernel.org>,
	"linux-security-module@vger.kernel.org" 
	<linux-security-module@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Andreas Gruenbacher <agruenba@redhat.com>
Subject: RE: [PATCH v5 09/12] evm: Allow setxattr() and setattr() for unmodified metadata
Date: Mon, 3 May 2021 15:32:52 +0000	[thread overview]
Message-ID: <b0bfaf2352b045dfaf443ae3af73b60e@huawei.com> (raw)
In-Reply-To: <cf12878833c82710ad4356e7d023cf51241f3cc8.camel@linux.ibm.com>

> From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> Sent: Monday, May 3, 2021 5:26 PM
> On Mon, 2021-05-03 at 15:11 +0000, Roberto Sassu wrote:
> > > From: Mimi Zohar [mailto:zohar@linux.ibm.com]
> > > Sent: Monday, May 3, 2021 3:00 PM
> > > On Wed, 2021-04-07 at 12:52 +0200, Roberto Sassu wrote:
> > >
> > > > diff --git a/security/integrity/evm/evm_main.c
> > > b/security/integrity/evm/evm_main.c
> > > > @@ -389,6 +473,11 @@ static int evm_protect_xattr(struct
> > > user_namespace *mnt_userns,
> > > >  	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> > > >  		return 0;
> > > >
> > > > +	if (evm_status == INTEGRITY_PASS_IMMUTABLE &&
> > > > +	    !evm_xattr_change(mnt_userns, dentry, xattr_name, xattr_value,
> > > > +			      xattr_value_len))
> > > > +		return 0;
> > > > +
> > >
> > > If the purpose of evm_protect_xattr() is to prevent allowing an invalid
> > > security.evm xattr from being re-calculated and updated, making it
> > > valid, INTEGRITY_PASS_IMMUTABLE shouldn't need to be conditional.
> Any
> > > time there is an attr or xattr change, including setting it to the
> > > existing value, the status flag should be reset.
> > >
> > > I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> > > prevent the file from being resigned.
> > >
> > > >  	if (evm_status != INTEGRITY_PASS)
> > > >  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA,
> > > d_backing_inode(dentry),
> > > >  				    dentry->d_name.name,
> > > "appraise_metadata",
> > >
> > > This would then be updated to if not INTEGRITY_PASS or
> > > INTEGRITY_PASS_IMMUTABLE.  The subsequent "return" would need to
> be
> > > updated as well.
> >
> > I agree on the first suggestion, to reduce the number of log messages.
> > For the second, if you meant that we should return 0 if the status is
> > INTEGRITY_PASS_IMMUTABLE, I thought we wanted to deny xattr
> > changes when there is an EVM portable signature.
> 
> Why?  I must be missing something.  As long as we're not relying on the
> cached status, allowing the file metadata to be updated shouldn't be an
> issue.

We may want to prevent accidental changes, for example.

Roberto

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