Re: [PATCH v2 00/15] ima: digest list feature

[Roberto Sassu]

On 11/9/2017 3:47 PM, Matthew Garrett wrote:
> On Thu, Nov 9, 2017 at 4:51 AM, Roberto Sassu <roberto.sassu@xxxxxxxxxx> wrote:
> > On 11/8/2017 4:48 PM, Matthew Garrett wrote:
> > > 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.

The default IMA policy measures files accessed from the initial ram
disk. It is easier to verify individual files, rather than the whole
image.


> > > > 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

You can have a look at ima_fs.c. If appraisal is in enforcing mode,
direct upload of a policy is not permitted. The kernel reads the policy,
calculates the digest, and verifies the signature.


> trustworthy at this point even in the absence of signatures. It seem > 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.

Then digest lists cannot be used alone, without an LSM. Also, verifiers
have to check the LSM policy to ensure that only the parser was able to
upload the digest lists.

Roberto

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