[Linux-ima-devel] [PATCH, RESEND 08/12] ima: added parser for RPM data type

[Mimi Zohar]

On Wed, 2017-08-09 at 11:15 +0200, Roberto Sassu wrote:
> On 8/2/2017 9:22 AM, James Morris wrote:
> > On Tue, 1 Aug 2017, Roberto Sassu wrote:
> >
> >> On 8/1/2017 12:27 PM, Christoph Hellwig wrote:
> >>> On Tue, Aug 01, 2017 at 12:20:36PM +0200, Roberto Sassu wrote:
> >>>> This patch introduces a parser for RPM packages. It extracts the digests
> >>>> from the RPMTAG_FILEDIGESTS header section and converts them to binary
> >>>> data
> >>>> before adding them to the hash table.
> >>>>
> >>>> The advantage of this data type is that verifiers can determine who
> >>>> produced that data, as headers are signed by Linux distributions vendors.
> >>>> RPM headers signatures can be provided as digest list metadata.
> >>>
> >>> Err, parsing arbitrary file formats has no business in the kernel.
> >>
> >> The benefit of this choice is that no actions are required for
> >> Linux distribution vendors to support the solution I'm proposing,
> >> because they already provide signed digest lists (RPM headers).
> >>
> >> Since the proof of loading a digest list is the digest of the
> >> digest list (included in the list metadata), if RPM headers are
> >> converted to a different format, remote attestation verifiers
> >> cannot check the signature.
> >>
> >> If the concern is security, it would be possible to prevent unsigned
> >> RPM headers from being parsed, if the PGP key type is upstreamed
> >> (adding in CC keyrings at vger.kernel.org).
> >
> > It's a security concern and also a layering violation, there should be no
> > need to parse package file formats in the kernel.
> 
> Parsing RPMs is not strictly necessary. Digests from the headers
> can be extracted and written to a new file using the compact data
> format (introduced with patch 7/12).
> 
> At boot time, IMA measures this file before digests are uploaded to the
> kernel. At this point, only files with unknown digest will be added
> to the measurement list. At verification time, verifiers recreate the
> measurement list by merging together the digests uploaded to the
> kernel with the unknown digests. Then, they verify the obtained list.
> 
> There are two ways to verify the digests: searching them in a reference
> database, or checking a signature. With the 'ima-sig' measurement list
> template, it is possible to verify signatures for each accessed file.
> With this patch set, it is possible to verify the signature of
> the file containing the digests uploaded to the kernel. If the data
> format changes, the signature cannot be verified.
> 
> To avoid this limitation, the parsers could be moved to a userspace
> tool which then uploads the parsed digests to the kernel. IMA would
> measure the original files. But, if the tool is compromised, it could
> load digests not included in the parsed files. With the current solution
> this problem does not arise because no changes can be done by userspace
> applications to the uploaded data while digests are parsed by IMA.
> 
> I could remove the RPM parser from the patch set for now.
> 
> Is the remaining part of the patch set ok, and is the explanation of
> what it does clear?

>From a trusted boot perspective, file measurements are added to the
measurement list, before access to the file is given.  The measurement
list contains ALL measurements, as defined by policy.  This patch set
changes that meaning to be all measurements, as defined by policy,
with the exception of those in a white list.

Changing the fundamental meaning of the measurement list is not
acceptable.  You could define a new securityfs file to differentiate
between the full measurement list and this abbreviated one.  But
before making this sort of change, I would prefer to address the
underlying problem - TPM peformance.

There are a couple of things that could be done to improve the TPM
driver performance, itself.  Once all of these options have been
pursued, we could then consider batching the measurements to the TPM,
meaning that the measurement list would still contain all the file
measurements, but instead of extending the TPM for each measurement, a
batched hash - a hash of a group of file measurements - would be
extended into the TPM.

Mimi

> > I'm not really clear on exactly how this patch series works.  Can you
> > provide a more concrete explanation of what steps would occur during boot
> > and attestation?
> >

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