Re: [PATCH v3 2/2] ima: add enforce-evm and log-evm modes to strictly check EVM status

[Roberto Sassu]

On 6/7/2019 5:08 PM, Mimi Zohar wrote:
> On Fri, 2019-06-07 at 16:40 +0200, Roberto Sassu wrote:
> > > On Thu, 2019-06-06 at 13:26 +0200, Roberto Sassu wrote:
>
> > > > Although this choice appears legitimate, it might not be suitable for
> > > > hardened systems, where the administrator expects that access is denied if
> > > > there is any error. An attacker could intentionally delete the EVM keys
> > > > from the system and set the file digest in security.ima to the actual file
> > > > digest so that the final appraisal status is INTEGRITY_PASS.
> > >
> > > Assuming that the EVM HMAC key is stored in the initramfs, not on some
> > > other file system, and the initramfs is signed, INTEGRITY_UNKNOWN
> > > would be limited to the rootfs filesystem.
> >
> > There is another issue. The HMAC key, like the public keys, should be
> > loaded when appraisal is disabled. This means that we have to create a
> > trusted key at early boot and defer the unsealing.
>
> There is no need for IMA to appraise the public key file signature,
> since the certificate is signed by a key on the builtin/secondary
> trusted keyring. ÂWithÂCONFIG_IMA_LOAD_X509 enabled, the public key
> can be loaded onto the IMA keyring with IMA-appraisal enabled, but
> without verifying the file signature.

Yes, but access to the files containing the master key and the EVM key
is denied if appraisal is enabled.

Roberto

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