Re: [DISCUSSION] svsm: attestation: if and how to authenticate the provided secret

On Fri, 2025-10-17 at 14:47 +0100, Daniel P. Berrangé wrote:
> On Fri, Oct 17, 2025 at 09:25:18AM -0400, James Bottomley wrote:
> > On Tue, 2025-10-14 at 21:33 -0400, Tyler Fanelli wrote:
> > > On 10/14/25 5:40 PM, James Bottomley wrote:
> > [...]
> > > > On this, how does the guest know it's pulled in the correct
> > > > image? If I were a malicious host, I could encrypt my own TPM
> > > > image, accept your attestation to a KBS I control and release a
> > > > key that decrypts my malicious image.  I think you give the
> > > > answer below:
> > > > 
> > > 
> > > We intend for the guest's rootfs to be sealed to the persistent
> > > TPM found in encrypted storage. The malicious host could switch
> > > the TPM, but that TPM wouldn't be able to unseal the rootfs.
> > > 
> > > This doesn't directly prevent the attack you mention above, but
> > > it would be a DoS.
> > 
> > Not necessarily.  If I control the host, I also control what you
> > boot in all stages.  If you simply take booting up as proof, then I
> > can substitute both the KBS and the boot system, so you'll boot
> > with my image but if you're running a trusted service in the VM,
> > it's now my service.  An examination of the launch measurement by a
> > trusted KBS would defeat this, which is why I substituted the KBS
> > as well.
> > 
> > The point being that there has to be validation on both sides.  If
> > you want a KBS to deliver a boot key by which your boot sequence
> > continuing is your validation, you still need to prove that you
> > contacted the correct KBS in the first place otherwise your launch
> > measurement might not actually have been validated.
> 
> For protection aganist that threat the guest owner would need to
> validate either the TPM identity, or the root FS identity, or both,
> once the VM is first accessed after booting.
> 
> In the root FS case, systemd measures the primary encryption key of
> any unlocked LUKS volume into PCR 15. If the user has prior knowledge
> of the expected hash they can use a TPM quote to validate that the
> root FS volume has not been substituted, and in turn that validates
> that the TPM state has not been substituted as the LUKS keyslot would
> have been sealed to the TPM.

I agree this would work ... but it's a who guards the guards problem
and the above solution needs another guard to guard the first one (the
KBS).

Another possible solution, which allows the KBS to be the only guard,
is to have the KBS launch the VM, meaning it knows it should get a
launch measurement back from the launched VM to release keys and can
shut it down if it doesn't.

Regards,

James