On Thu, Nov 9, 2017 at 4:51 AM, Roberto Sassu [off-list ref] wrote:

On 11/8/2017 4:48 PM, Matthew Garrett wrote:

quoted

The code doing the parsing is in the initramfs, which has already been
measured at boot time. You can guarantee that it's being done by
trusted code.

The parser can be executed in the initial ram disk, but everything
accessed before the parser is executed will be measured/appraised
without digest lists. To do signature-based remote attestation, where
the verification consists on checking the signature of digests of
measured files, it would be necessary to sign systemd, libraries,
everything accessed before the parser, and the parser. If RPM headers
are parsed by the kernel, measurement/appraisal will be done directly
with digest lists.

There's no need to have a policy that measures those files, because
they're part of the already-measured initramfs. Just set the IMA
policy after you've loaded the digest list.

quoted

quoted

The main problem is that the digest list measurement, performed when the
parser accesses the file containing the RPM header, might not reflect
what IMA uses for digest lookup.

Why not?

I assumed you wanted to measure digest lists only at the time they are
read by the parser, and not when they are read by IMA. If instead digest
lists are verified again after conversion, the new workflow should be:

1) the kernel parses digest list metadata before systemd is executed
2) the kernel verifies the signature of digest lists (RPM headers) and
   add the digest of digest lists to the hash table, so that appraisal
   succeeds
3) systemd (with file signature) is executed
4) the parser (with file signature) is executed
5) the parser reads and converts the digest lists to the generic format,
   and writes them to a tmpfs filesystem
6) the parser generates a new digest list metadata file with the path of
   converted digest lists and sends the path of the new metadata to IMA
7) IMA reads the generic digest lists

The measurement list should look like:

10 <digest> ima-sig <digest> boot_aggregate
10 <digest> ima-sig <digest> /etc/ima/digest_lists/metadata
10 <digest> ima-sig <digest> /usr/lib/systemd/systemd <signature>
...
10 <digest> ima-sig <digest> <parser> <signature>
10 <digest> ima-sig <digest> /tmp/metadata


If parsing of RPM headers is done by the kernel, the measurement list
will look like:

10 <digest> ima-ng <digest> boot_aggregate
10 <digest> ima-ng <digest> /etc/ima/digest_lists/metadata


A built-in policy should enable appraisal of tmpfs. If not, patch 11/15
disables digest lookup for appraisal. Since generic digest lists will
have a security.ima extended attribute (they are mutable files),
appraisal verification will succeed.

With this solution, digital signatures cannot be required, because
generic digest lists will have a HMAC. For appraisal, it becomes
necessary to ensure that only digest lists written by the parser can be
processed by IMA.

This seems very over-complicated, and it's unclear why the kernel
needs to open the file itself. You *know* that all of userland is
trustworthy at this point even in the absence of signatures. It seems
reasonable to provide a interface that allows userland to pass a
digest list to the kernel, in the same way that userland can pass an
IMA policy to the kernel. You can then restrict access to that
interface via an LSM.
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