Re: [PATCH 07/11] evm: Set IMA_CHANGE_XATTR/ATTR bit if EVM_ALLOW_METADATA_WRITES is set

[Mimi Zohar]

On Tue, 2020-09-01 at 09:08 +0000, Roberto Sassu wrote:
> > From: Mimi Zohar [mailto:zohar@xxxxxxxxxxxxx]
> > 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evm_status is no longer valid.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.

Mimi