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

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Message-ID: <06edfc9f779447b9b93f26628327d1e5@huawei.com>
Date:   Mon, 3 May 2021 14:48:50 +0000
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     Mimi Zohar <zohar@...ux.ibm.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

> 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.

The status is always reset if evm_protect_xattr() returns 0. This does not
change.

Not making INTEGRITY_PASS_IMMUTABLE conditional would cause issues.
Suppose that the status is INTEGRITY_FAIL. Writing the same xattr would
cause evm_protect_xattr() to return 0 and the HMAC to be updated.

> I'm wondering if making INTEGRITY_PASS_IMMUTABLE conditional would
> prevent the file from being resigned.

INTEGRITY_FAIL_IMMUTABLE should be enough to continue the
operation.

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli

> >  	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.
> 
> thanks,
> 
> Mimi