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

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Date:   Tue, 11 May 2021 14:54:58 +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 4:41 PM
> On Tue, 2021-05-11 at 14:21 +0000, Roberto Sassu wrote:
> > >
> > > 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.
> >
> > 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.
> 
> I understand we might want to prevent accidental or malicious changes,
> but that isn't the purpose of this patch set.  The patch description
> would also need to be updated to reflect the real purpose.

We would be changing the expectation that metadata changes
are denied, which was defined with the original patches.

I would prefer to keep the current behavior, but if your suggestion
is to allow metadata changes, I will modify the patch set.

Roberto

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

> thanks,
> 
> Mimi