linux-kernel - RE: [PATCH 07/11] evm: Set IMA_CHANGE_XATTR/ATTR bit if EVM_ALLOW_METADATA

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Date:   Tue, 1 Sep 2020 11:41:56 +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>,
        Silviu Vlasceanu <Silviu.Vlasceanu@...wei.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH 07/11] evm: Set IMA_CHANGE_XATTR/ATTR bit if
 EVM_ALLOW_METADATA_WRITES is set

> From: Mimi Zohar [mailto:zohar@...ux.ibm.com]
> Sent: Tuesday, September 1, 2020 1:05 PM
> On Tue, 2020-09-01 at 09:08 +0000, Roberto Sassu wrote:
> > > From: Mimi Zohar [mailto:zohar@...ux.ibm.com]
> > > Sent: Monday, August 24, 2020 2:18 PM
> > > On Thu, 2020-06-18 at 18:04 +0200, Roberto Sassu wrote:
> > > > When EVM_ALLOW_METADATA_WRITES is set, EVM allows any
> operation
> > > on
> > > > metadata. Its main purpose is to allow users to freely set metadata
> when
> > > > they are protected by a portable signature, until the HMAC key is
> loaded.
> > > >
> > > > However, IMA is not notified about metadata changes and, after the
> first
> > > > appraisal, always allows access to the files without checking metadata
> > > > again.
> > >
> > > ^after the first successful appraisal
> > > >
> > > > This patch checks in evm_reset_status() if EVM_ALLOW_METADATA
> > > WRITES is
> > > > enabled and if it is, sets the IMA_CHANGE_XATTR/ATTR bits
> depending on
> > > the
> > > > operation performed. At the next appraisal, metadata are revalidated.
> > >
> > > EVM modifying IMA bits crosses the boundary between EVM and IMA.
> > > There
> > > is already an IMA post_setattr hook.  IMA could reset its own bit
> > > there.  If necessary EVM could export as a function it's status info.
> >
> > I wouldn't try to guess in IMA when EVM resets its status. We would have
> > to duplicate the logic to check if an EVM key is loaded, if the passed xattr
> > is a POSIX ACL, ...
> 
> Agreed, but IMA could call an EVM function.
> 
> >
> > I think it is better to set a flag, maybe a new one, directly in EVM, to notify
> > the integrity subsystem that iint->evm_status is no longer valid.
> >
> > If the EVM flag is set, IMA would reset the appraisal flags, as it uses
> > iint->evm_status for appraisal. We can consider to reset also the measure
> > flags when we have a template that includes file metadata.
> 
> When would IMA read the EVM flag?   Who would reset the flag?  At what
> point would it be reset?   Just as EVM shouldn't be resetting the IMA
> flag, IMA shouldn't be resetting the EVM flag.

IMA would read the flag in process_measurement() and behave similarly
to when it processes IMA_CHANGE_ATTR. The flag would be reset by
evm_verify_hmac().

Roberto

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