Messages in this thread | | | Date | Fri, 12 Nov 2021 21:53:28 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support |
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On Fri, Nov 12, 2021 at 08:37:59PM +0000, Sean Christopherson wrote: > Let userspace decide what is mapped shared and what is mapped private.
With "userspace", you mean the *host* userspace?
> 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.
That SIGSEGV is generated by the host kernel, I presume, after it checks whether the memory belongs to the guest?
> - if kernel accesses guest private memory, it does BUG/panic/oops[*]
If *it* is the host kernel, then you probably shouldn't do that - otherwise you just killed the host kernel on which all those guests are running.
> - if guest accesses memory with the incorrect C/SHARED-bit, it gets killed.
Yah, that's the easy one.
> This is the direction KVM TDX support is headed, though it's obviously still a WIP. > > 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.
I like the explicit nature of this but devil's in the detail and I'm no virt guy...
> 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.
Yah, the #MC thing sounds like someone didn't think things through. ;-\
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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