Re: AMD SEV-SNP/Intel TDX: validation of memory pages

[Andi Kleen <ak@linux.intel.com>]

> > Is this something the TDX thread model covers? A malicous HV and a TDX
> > guest co-operating to bring down the guest kernel.
> 
> If the guest is not malicous, and you have a valid user-stack in the
> SYSCALL gap, then it depends on whether SMAP is active. I guess it
> usually is, in which case the #VE would be promoted to a #DF to kill the
> machine.

An malicious user process in the guest could set user RSP to a kernel address.

But yes without that it's probably not a problem due to SMAP.

It really boils down if we consider this combination of "malicious
hypervisor and malicious user process together" to be a threat.

At least in the classical TDX model malicious guest process was
out of scope. So considering it would be a new requirement.

For the kernel exit we could handle it by checking if the 
RSP address is in the kernel, and killing the process
before switching the stack.  It's just a problem for the entry,
which probably would really need an IST.

For the IST we would need to handle two level nesting, but I presume that
would be possible. It should be much simpler than the NMI nesting
with just a single stack switch.

I think the IST solution should at least be explored before
dismissing it. It might be simpler than anything else (like
using new APIs)

-Andi