Re: [PATCH Part2 v6 35/49] KVM: SVM: Remove the long-lived GHCB host map

[Peter Gonda <pgonda@google.com>]

On Thu, Jul 7, 2022 at 2:31 PM Kalra, Ashish <Ashish.Kalra@amd.com> wrote:
>
> [AMD Official Use Only - General]
>
> Hello Peter,
>
> >> >There is a perf cost to this suggestion but it might make accessing
> >> >the GHCB safer for KVM. Have you thought about just using
> >> >kvm_read_guest() or copy_from_user() to fully copy out the GCHB into a KVM owned buffer, then copying it back before the VMRUN. That way the KVM doesn't need to guard against page_state_changes on the GHCBs, that could be a perf ?>>improvement in a follow up.
> >>
> >> Along with the performance costs you mentioned, the main concern here
> >> will be the GHCB write-back path (copying it back) before VMRUN: this
> >> will again hit the issue we have currently with
> >> kvm_write_guest() / copy_to_user(), when we use it to sync the scratch
> >> buffer back to GHCB. This can fail if guest RAM is mapped using huge-page(s) and RMP is 4K. Please refer to the patch/fix mentioned below, kvm_write_guest() potentially can fail before VMRUN in case of SNP :
> >>
> >> commit 94ed878c2669532ebae8eb9b4503f19aa33cd7aa
> >> Author: Ashish Kalra <ashish.kalra@amd.com>
> >> Date:   Mon Jun 6 22:28:01 2022 +0000
> >>
> >>     KVM: SVM: Sync the GHCB scratch buffer using already mapped ghcb
> >>
> >>    Using kvm_write_guest() to sync the GHCB scratch buffer can fail
> >>     due to host mapping being 2M, but RMP being 4K. The page fault handling
> >>     in do_user_addr_fault() fails to split the 2M page to handle RMP fault due
> >>     to it being called here in a non-preemptible context. Instead use
> >>     the already kernel mapped ghcb to sync the scratch buffer when the
> >>     scratch buffer is contained within the GHCB.
>
> >Ah I didn't see that issue thanks for the pointer.
>
> >The patch description says "When SEV-SNP is enabled the mapped GPA needs to be protected against a page state change." since if the guest were to convert the GHCB page to private when the host is using the GHCB the host could get an RMP violation right?
>
> Right.
>
> >That RMP violation would cause the host to crash unless we use some copy_to_user() type protections.
>
> As such copy_to_user() will only swallow the RMP violation and return failure, so the host can retry the write.
>
> > I don't see anything mechanism for this patch to add the page state change protection discussed. Can't another vCPU still convert the GHCB to private?
>
> We do have the protections for GHCB getting mapped to private specifically, there are new post_{map|unmap}_gfn functions added to verify if it is safe to map
> GHCB pages. There is a PSC spinlock added which protects again page state change for these mapped pages.
> Below is the reference to this patch:
> https://lore.kernel.org/lkml/cover.1655761627.git.ashish.kalra@amd.com/T/#mafcaac7296eb9a92c0ea58730dbd3ca47a8e0756
>
> But do note that there is protection only for GHCB pages and there is a need to add generic post_{map,unmap}_gfn() ops that can be used to verify
> that it's safe to map a given guest page in the hypervisor. This is a TODO right now and probably this is something which UPM can address more cleanly.

Thank you Ashish. I had missed that.

Can you help me understand why its OK to use kvm_write_guest() for the
|snp_certs_data| inside of snp_handle_ext_guest_request() in patch
42/49? I would have thought we'd have the same 2M vs 4K mapping
issues.

>
> >I was wrong about the importance of this though seanjc@ walked me through how UPM will solve this issue so no worries about this until the series is rebased on to UPM.
>
> Thanks,
> Ashish