Re: [PATCH v6 08/11] evm: Allow setxattr() and setattr() for unmodified metadata

From: Mimi Zohar <zohar@linux.ibm.com>
To: Roberto Sassu <roberto.sassu@huawei.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-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v6 08/11] evm: Allow setxattr() and setattr() for unmodified metadata
Date: Tue, 11 May 2021 16:00:29 -0400	[thread overview]
Message-ID: <234ccd701bec53b979015fdd257e48400cd68759.camel@linux.ibm.com> (raw)
In-Reply-To: <0d0fcd3619e64bb9aaf1656ef066d043@huawei.com>

On Tue, 2021-05-11 at 14:54 +0000, Roberto Sassu wrote:
> > 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.

Please re-write the patch description appropriately.

thanks,

Mimi