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

From: Roberto Sassu <roberto.sassu@huawei.com>
To: James Morris <jmorris@namei.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org, keyrings@vger.kernel.org,
	linux-ima-devel@lists.sourceforge.net
Subject: Re: [Linux-ima-devel] [PATCH, RESEND 08/12] ima: added parser for RPM data type
Date: Wed, 09 Aug 2017 09:15:32 +0000	[thread overview]
Message-ID: <0506050f-c4f1-1b36-a25b-c5418607906d@huawei.com> (raw)
In-Reply-To: <alpine.LRH.2.20.1708021716440.1117@namei.org>

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@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?

Thanks

Roberto

> 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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