Re: [RFCv2 00/16] KVM protected memory extension

[Kirill A. Shutemov]

On Wed, Oct 21, 2020 at 04:46:48PM +0200, Vitaly Kuznetsov wrote:
> "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes:
> 
> > On Tue, Oct 20, 2020 at 09:46:11AM +0200, Vitaly Kuznetsov wrote:
> >> "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes:
> >> 
> >> > == Background / Problem ==
> >> >
> >> > There are a number of hardware features (MKTME, SEV) which protect guest
> >> > memory from some unauthorized host access. The patchset proposes a purely
> >> > software feature that mitigates some of the same host-side read-only
> >> > attacks.
> >> >
> >> >
> >> > == What does this set mitigate? ==
> >> >
> >> >  - Host kernel ”accidental” access to guest data (think speculation)
> >> >
> >> >  - Host kernel induced access to guest data (write(fd, &guest_data_ptr, len))
> >> >
> >> >  - Host userspace access to guest data (compromised qemu)
> >> >
> >> >  - Guest privilege escalation via compromised QEMU device emulation
> >> >
> >> > == What does this set NOT mitigate? ==
> >> >
> >> >  - Full host kernel compromise.  Kernel will just map the pages again.
> >> >
> >> >  - Hardware attacks
> >> >
> >> >
> >> > The second RFC revision addresses /most/ of the feedback.
> >> >
> >> > I still didn't found a good solution to reboot and kexec. Unprotect all
> >> > the memory on such operations defeat the goal of the feature. Clearing up
> >> > most of the memory before unprotecting what is required for reboot (or
> >> > kexec) is tedious and error-prone.
> >> > Maybe we should just declare them unsupported?
> >> 
> >> Making reboot unsupported is a hard sell. Could you please elaborate on
> >> why you think that "unprotect all" hypercall (or rather a single
> >> hypercall supporting both protecting/unprotecting) defeats the purpose
> >> of the feature?
> >
> > If guest has some data that it prefers not to leak to the host and use the
> > feature for the purpose, share all the memory to get through reboot is a
> > very weak point.
> >
> 
> My point that if it knows that there's something sensitive in its
> memory it should clean it up even today without your feature before
> rebooting to an unknown target.

It's unrealistic to expect everybody to do the right thing.

> >> clean up *all* its memory upon reboot, however:
> >> - It may only clean up the most sensitive parts. This should probably be
> >> done even without this new feature and even on bare metal (think about
> >> next boot target being malicious).
> >> - The attack window shrinks significantly. "Speculative" bugs require
> >> time to exploit and it will only remain open until it boots up again
> >> (few seconds).
> >
> > Maybe it would be cleaner to handle reboot in userspace? If we got the VM
> > rebooted, just reconstruct it from scratch as if it would be new boot.
> 
> We are definitely not trying to protect against malicious KVM so maybe
> we can do the cleanup there (when protection was enabled) so we can
> unprotect everything without risk of a leak?

Do you have any particular codepath in mind? I didn't find anything
suitable so far.

-- 
 Kirill A. Shutemov