Re: [PATCH v8 00/26] Enable CET Virtualization

[Yang, Weijiang]

On 1/5/2024 5:10 AM, Edgecombe, Rick P wrote:
> On Thu, 2024-01-04 at 15:11 +0800, Yang, Weijiang wrote:
[...]
> > > My understanding is that the KVM emulator kind of does what it has
> > > to
> > > keep things running, and isn't expected to emulate every possible
> > > instruction. With CET though, it is changing the behavior of
> > > existing
> > > supported instructions. I could imagine a guest could skip over CET
> > > enforcement by causing an MMIO exit and racing to overwrite the
> > > exit-
> > > causing instruction from a different vcpu to be an indirect
> > > CALL/RET,
> > > etc.
> > Can you elaborate the case? I cannot figure out how it works.
> The point that it should be possible for KVM to emulate call/ret with
> CET enabled. Not saying the specific case is critical, but the one I
> used as an example was that the KVM emulator can (or at least in the
> not too distant past) be forced to emulate arbitrary instructions if
> the guest overwrites the instruction between the exit and the SW fetch
> from the host.
>
> The steps are:
> vcpu 1                         vcpu 2
> -------------------------------------
> mov to mmio addr
> vm exit ept_misconfig
>                                 overwrite mov instruction to call %rax
> host emulator fetches
> host emulates call instruction
>
> So then the guest call operation will skip the endbranch check. But I'm
> not sure that there are not less exotic cases that would run across it.
> I see a bunch of cases where write protected memory kicks to the
> emulator as well. Not sure the exact scenarios and whether this could
> happen naturally in races during live migration, dirty tracking, etc.
> Again, I'm more just asking the exposure and thinking on it.

Now I get your points, I didn't think of exposure from guest and just thought of the
normal execution flow in guest, so I said let guest handle #CP directly.

Yes, I think we need to take these cases into account, as Sean suggested in following
replies, stopping emulation JMP/CALL/RET etc. instructions when guest CET is enabled
is effective and simple, I'll investigate the emulator code.

Thanks for raising the concerns!