Re: [RFD] linux-firmware key arrangement for firmware signing

Mimi Zohar
2015-05-20T02:06:56Z

On Tue, 2015-05-19 at 18:29 -0700, Andy Lutomirski wrote:
> On Tue, May 19, 2015 at 6:06 PM, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> > On Tue, 2015-05-19 at 17:22 -0700, Luis R. Rodriguez wrote:
> >> On Tue, May 19, 2015 at 4:37 PM, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> >> > On Wed, 2015-05-20 at 00:19 +0200, Luis R. Rodriguez wrote:
> >> >> On Tue, May 19, 2015 at 05:48:37PM -0400, Mimi Zohar wrote:
> >> >> > On Tue, 2015-05-19 at 22:02 +0200, Luis R. Rodriguez wrote:
> >
> >> > In this case, not only is there a
> >> > security hook, but the IMA hook exists as well. To appraise firmware,
> >> > add a line to the IMA policy containing "appraise func=FIRMWARE_CHECK".
> >> > Similarly, to add a measurement to the measurement list, add a line to
> >> > the IMA policy containing "measure func=FIRMWAE_CHECK".
> >>
> >> I have a series of reasons find IMA unsuitable for the current goals at hand:
> >>
> >> 1) IMA is a pretty big kitchen sink, we want this to work well for
> >> even embedded systems, or architectures that do not have or require
> >> TPMs
> >
> > There are different aspects to IMA. One aspect is collecting file
> > measurements and extending the TPM with those measurements. The other
> > aspect is appraising file integrity. For that aspect, IMA-appraisal
> > does not use a TPM.
> >
> >> 2) The appraisal is also done for to account for a specific state of
> >> affairs, you appraise to the user of the integrity of the system at a
> >> specific point in time,
> >
> > True, IMA can be used to attest to the integrity of a system.
> >
> >> firmware signing can provide integrity /
> >> authorship vetting of files directly from the authors.
> >
> > It can also be used to appraise the integrity of a file, be it an
> > executable, a kernel module, configuration file or firmware in a
> > consistent manor, based on a file hash or signature.
> >
> >> In the case of
> >> regulatory.bin that was the whole point of it, and firmware signing as
> >> is being provided is intended to generalize that but by sharing code
> >> in-kernel with module signing infrastructure
> >
> > The underlying code used to verify the file signatures is the same.
> > The difference being where/how the file signatures are stored and which
> > keys to trust.
> >
> >> I am in hopes some others might be able to chime in more on point 2) here.
> >>
> >> Don't get me wrong IMA is nice, but its a big chunky requirement to
> >> have, more than what module signing provides and what it requires
> >> today to replace subsystem file signing requirements.
> >
> >> Now, LSM hooks -- that's more aligned with something we can start IMHO
> >> reasonably arguing we should shift module signing code to be punted
> >> into. But I've heard stories of LSM having issues with some virtual
> >> environments, and LSM stacking is also pretty new, and IMHO that'd be
> >> one way to compartmentalize all this module signing code. IMHO that
> >> *should happen* but can only be taken seriously once LSM stacking is
> >> merged in and baked. Its not, but I'm excited for it.
> >
> > Have you even looked at IMA-appraisal?
>
> I just looked extremely briefly. It seems to be much simpler than the
> PKCS#7 thing. OTOH, it seems to hardcode some rather scary
> assumptions that it's using RSA in digsig_asymmetric.c, specifically:
>
> pks.rsa.s = mpi_read_raw_data(hdr->sig, siglen);
> This bit in ima_appraise_measurement ignores trailing junk. Why?

The same field is used to store either an MD5 or a SHA1 hash. The hash
length is dependent on the hash algorithm

> if (xattr_len - sizeof(xattr_value->type) - hash_start >=
> iint->ima_hash->length)
> /* xattr length may be longer. md5 hash in previous
> version occupied 20 bytes in xattr, instead of 16
> */
> rc = memcmp(&xattr_value->digest[hash_start],
> iint->ima_hash->digest,
> iint->ima_hash->length);
>
> I got confused around here:
>
> /* Replace RSA with HMAC if not mounted readonly and
> * not immutable
> */
> if (!IS_RDONLY(dentry->d_inode) &&
> !IS_IMMUTABLE(dentry->d_inode))
> evm_update_evmxattr(dentry, xattr_name,
> xattr_value,
> xattr_value_len);
>
> Huh?

EVM signatures are replaced with a keyed HMAC.

>
> Anyway, AFAICT IMA is about tracking the integrity of an FS that's
> being actively modified, not about distributing signed things.

Files that are signed are considered immutable and do not change
(ima_appraise_measurement: case EVM_IMA_XATTR_DIGSIG).

Fin posted RPM patches to store file signatures in the RPM header.
Similarly, I posted patches to include file signatures in deb packages.
(We still need to upstream these patches.)

> Also,
> I couldn't spot what part of IMA detects an attacker replacing one
> signed file with a different one from the same filesystem but a
> different name.

Perhaps you're referring to EVM?

Mimi