Messages in this thread | | | Subject | Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support | From | Brijesh Singh <> | Date | Mon, 15 Nov 2021 10:18:16 -0600 |
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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
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