linux-kernel - Re: [PATCH v5 09/12] evm: Allow setxattr() and setattr() for unmodified metadata

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Message-ID: <d48e3595d0a39cc0ac82519c1c444eeacc8b8c58.camel@linux.ibm.com>
Date:   Mon, 03 May 2021 11:48:04 -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-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andreas Gruenbacher <agruenba@...hat.com>
Subject: Re: [PATCH v5 09/12] evm: Allow setxattr() and setattr() for
 unmodified metadata

On Mon, 2021-05-03 at 15:32 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@...ux.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@...ux.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.

Let's keep it simple, getting the basics working properly first.  Then
we can decide if this is something that we really want/need to defend
against.

thanks,

Mimi