Re: [PATCH v7 00/16] EVM

Mimi Zohar
2011-06-30T03:51:54Z

On Wed, 2011-06-29 at 21:57 -0400, Kyle Moffett wrote:
> On Wed, Jun 29, 2011 at 19:42, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> > On Wed, 2011-06-29 at 16:53 -0400, Kyle Moffett wrote:
> >> On Wed, Jun 29, 2011 at 15:50, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> >> > Discretionary Access Control(DAC) and Mandatory Access Control(MAC) can
> >> > protect the integrity of a running system from unauthorized changes. When
> >> > these protections are not running, such as when booting a malicious OS,
> >> > mounting the disk under a different operating system, or physically moving
> >> > the disk to another system, an "offline" attack is free to read and write
> >> > file data/metadata.
> >> >
> >> > Extended Verification Module(EVM) detects offline tampering of the security
> >> > extended attributes (e.g. security.selinux, security.SMACK64, security.ima),
> >> > which is the basis for LSM permission decisions and, with the IMA-appraisal
> >> > patchset, integrity appraisal decisions. This patchset provides the framework
> >> > and an initial method to detect offline tampering of the security extended
> >> > attributes. The initial method maintains an HMAC-sha1 across a set of
> >> > security extended attributes, storing the HMAC as the extended attribute
> >> > 'security.evm'. To verify the integrity of an extended attribute, EVM exports
> >> > evm_verifyxattr(), which re-calculates the HMAC and compares it with the
> >> > version stored in 'security.evm'. Other methods of validating the integrity
> >> > of a file's metadata will be posted separately (eg. EVM-digital-signatures).
> >>
> >> Hmm, I'm not sure that this design actually provides the protection that
> >> you claim it does.
> >>
> >> Specifically, you don't actually protect the on-disk data-structures that
> >> are far more vulnerable to malicious modification than the actual *values*
> >> of the extended attributes themselves.
> >
> > True, EVM only protects the file metadata. The patch description says,
> >
> > While this patchset does authenticate the security xattrs, and
> > cryptographically binds them to the inode, coming extensions
> > will bind other directory and inode metadata for more complete
> > protection.
> >
> > It should have said, "bind other directory, inode data and inode
> > metadata."
> >
> > In particular, IMA-appraisal stores the file data's hash as the
> > security.ima xattr, which is EVM protected. Other methods, such as
> > digital signatures, could be used instead of the file's hash, to
> > additionally provide authenticity.
>
> The problem is that your *design* assumes that the filesystem itself is
> valid, but your stated threat model assumes that the attacker has offline
> access to the filesystem, an explicit contradiction.
>
> There have been numerous cases in the past where a corrupt or invalid
> filesystem causes kernel panics or even exploitable overflows or memory
> corruption; see the history of the "fsfuzzer" tool for more information.
>
> Furthermore, if the attacker can intentionally cause data extent or inode
> extended attribute aliasing (shared space-on-disk) between different
> files then your entire security model falls flat.
>
> So if you assume the attacker has raw access to the underlying filesystem
> then you MUST authenticate *all* of the low-level filesystem data,
> including the "implicit" metadata of allocation tables, extents, etc.
>
> Cheers,
> Kyle Moffett

Assuming someone does modify the underlying filesystem, how does that
break the security model? The purpose of EVM/IMA-appraisal is not to
prevent files offline from being modified, but to detect if/when it
occurs and to enforce file integrity.

Mimi