linux-kernel - RE: [PATCH v6 08/11] evm: Allow setxattr() and setattr() for unmodified metadata

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Message-ID: <c281b39bdbaa4b5ab921a2e9cece83b4@huawei.com>
Date:   Tue, 11 May 2021 14:21:28 +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-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        kernel test robot <lkp@...el.com>
Subject: RE: [PATCH v6 08/11] evm: Allow setxattr() and setattr() for
 unmodified metadata

> From: Mimi Zohar [mailto:zohar@...ux.ibm.com]
> Sent: Tuesday, May 11, 2021 4:12 PM
> Hi Roberto,
> 
> On Wed, 2021-05-05 at 13:33 +0200, Roberto Sassu wrote:
> > With the patch to allow xattr/attr operations if a portable signature
> > verification fails, cp and tar can copy all xattrs/attrs so that at the
> > end of the process verification succeeds.
> >
> > However, it might happen that the xattrs/attrs are already set to the
> > correct value (taken at signing time) and signature verification succeeds
> > before the copy has completed. For example, an archive might contains files
> > owned by root and the archive is extracted by root.
> >
> > Then, since portable signatures are immutable, all subsequent operations
> > fail (e.g. fchown()), even if the operation is legitimate (does not alter
> > the current value).
> >
> > This patch avoids this problem by reporting successful operation to user
> > space when that operation does not alter the current value of xattrs/attrs.
> 
> I must be missing something.  If both the IMA and EVM status flags are
> reset after xattr or attr modification, do we really need to prevent
> any metadata - same or different - changes?  Both evm_protect_xattr()
> and evm_inode_setattr() would need to be modified to allow
> INTEGRITY_PASS_IMMUTABLE.

Hi Mimi

yes, given that the IMA and EVM flags are reset, it should not be
a problem to allow changes. However, I think it is useful to keep
the current behavior. For example, it would prevent an accidental
change of the SELinux label during the relabeling process.

Roberto

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

> thanks,
> 
> Mimi