Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

[Brijesh Singh]

On 11/12/21 2:37 PM, Sean Christopherson wrote:
> On Fri, Nov 12, 2021, Borislav Petkov wrote:
> > On Fri, Nov 12, 2021 at 07:48:17PM +0000, Sean Christopherson wrote:
> > > Yes, but IMO inducing a fault in the guest because of _host_ bug is wrong.

In the automatic change proposal, both the the host and a guest bug will 
cause a guest to get the #VC and then the guest can decide whether it 
wants to proceed or terminate. If it chooses to move, it can poison the 
page and log it for future examination.

> > What do you suggest instead?
>
> Let userspace decide what is mapped shared and what is mapped private.  The kernel
> and KVM provide the APIs/infrastructure to do the actual conversions in a thread-safe
> fashion and also to enforce the current state, but userspace is the control plane.
>
> It would require non-trivial changes in userspace if there are multiple processes
> accessing guest memory, e.g. Peter's networking daemon example, but it _is_ fully
> solvable.  The exit to userspace means all three components (guest, kernel,
> and userspace) have full knowledge of what is shared and what is private.  There
> is zero ambiguity:
>
>    - if userspace accesses guest private memory, it gets SIGSEGV or whatever.
>    - if kernel accesses guest private memory, it does BUG/panic/oops[*]
>    - if guest accesses memory with the incorrect C/SHARED-bit, it gets killed.
>
> This is the direction KVM TDX support is headed, though it's obviously still a WIP.

Just curious, in this approach, how do you propose handling the host 
kexec/kdump? If a kexec/kdump occurs while the VM is still active, the 
new kernel will encounter the #PF (RMP violation) because some pages are 
still marked 'private' in the RMP table.



> And ideally, to avoid implicit conversions at any level, hardware vendors' ABIs
> define that:
>
>    a) All convertible memory, i.e. RAM, starts as private.
>    b) Conversions between private and shared must be done via explicit hypercall.
>
> Without (b), userspace and thus KVM have to treat guest accesses to the incorrect
> type as implicit conversions.
>
> [*] Sadly, fully preventing kernel access to guest private is not possible with
>      TDX, especially if the direct map is left intact.  But maybe in the future
>      TDX will signal a fault instead of poisoning memory and leaving a #MC mine.

thanks