Re: [PATCH] security: Fix IMA Kconfig for dependencies on ARM64

[Mimi Zohar]

On Mon, 2018-03-12 at 15:30 -0700, James Bottomley wrote:
> On Mon, 2018-03-12 at 17:53 -0400, Mimi Zohar wrote:
[...]
> > - This use case, when the TPM is not builtin and unavailable before
> > IMA is initialized.
> > 
> > I would classify this use case as an IMA testing/debugging
> > environment, when it cannot, for whatever reason, be builtin the
> > kernel or initialized before IMA.
> > 
> > From Dave Safford:
> > ÂÂÂÂFor the TCG chain of trust to have any meaning, all files have to
> > ÂÂÂÂbe measured and extended into the TPM before they are accessed.
> > If
> > ÂÂÂÂthe TPM driver is loaded after any unmeasured file, the chain is
> > ÂÂÂÂbroken, and IMA is useless for any use case or any threat model.
> 
> I don't think this is quite the correct characterisation. ÂIn principle
> the kernel could also touch the files before IMA is loaded. ÂHowever,
> we know from the way the kernel operates that it doesn't. ÂWe basically
> trust that the kernel measurement tells us this. ÂThe same thing can be
> made to apply to the initrd.

With the builtin "tcb" policy, IMA-measurement is enabled from the
very beginning. ÂAfterwards, the system can transition to a custom
policy based on finer grain LSM labels, which aren't available on
boot.

> The key question is not whether the component could theoretically
> access the files but whether it actually does so.

As much as you might think you know what is included in the initramfs,
IMA-measurement is your safety net, including everything accessed in
the TCB.

Mimi