On 5/20/21 3:22 PM, Sean Christopherson wrote: > On Thu, May 20, 2021, Sean Christopherson wrote: >> On Thu, May 20, 2021, Sean Christopherson wrote: >>> On Mon, May 17, 2021, Tom Lendacky wrote: >>>> On 5/14/21 6:06 PM, Peter Gonda wrote: >>>>> On Fri, May 14, 2021 at 1:22 PM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote: >>>>>> >>>>>> Currently, an SEV-ES guest is terminated if the validation of the VMGEXIT >>>>>> exit code and parameters fail. Since the VMGEXIT instruction can be issued >>>>>> from userspace, even though userspace (likely) can't update the GHCB, >>>>>> don't allow userspace to be able to kill the guest. >>>>>> >>>>>> Return a #GP request through the GHCB when validation fails, rather than >>>>>> terminating the guest. >>>>> >>>>> Is this a gap in the spec? I don't see anything that details what >>>>> should happen if the correct fields for NAE are not set in the first >>>>> couple paragraphs of section 4 'GHCB Protocol'. >>>> >>>> No, I don't think the spec needs to spell out everything like this. The >>>> hypervisor is free to determine its course of action in this case. >>> >>> The hypervisor can decide whether to inject/return an error or kill the guest, >>> but what errors can be returned and how they're returned absolutely needs to be >>> ABI between guest and host, and to make the ABI vendor agnostic the GHCB spec >>> is the logical place to define said ABI. >>> >>> For example, "injecting" #GP if the guest botched the GHCB on #VMGEXIT(CPUID) is >>> completely nonsensical. As is, a Linux guest appears to blindly forward the #GP, >>> which means if something does go awry KVM has just made debugging the guest that >>> much harder, e.g. imagine the confusion that will ensue if the end result is a >>> SIGBUS to userspace on CPUID. >>> >>> There needs to be an explicit error code for "you gave me bad data", otherwise >>> we're signing ourselves up for future pain. >> >> More concretely, I think the best course of action is to define a new return code >> in SW_EXITINFO1[31:0], e.g. '2', with additional information in SW_EXITINFO2. >> >> In theory, an old-but-sane guest will interpret the unexpected return code as >> fatal to whatever triggered the #VMGEXIT, e.g. SIGBUS to userspace. Unfortunately >> Linux isn't sane because sev_es_ghcb_hv_call() assumes any non-'1' result means >> success, but that's trivial to fix and IMO should be fixed irrespective of where >> this goes. > > One last thing (hopefully): Erdem pointed out that if the GCHB GPA (or any > derferenced pointers within the GHCB) is invalid or is set to a private GPA > (mostly in the context of SNP) then the VMM will likely have no choice but to > kill the guest in response to #VMGEXIT. > > It's probably a good idea to add a blurb in one of the specs explicitly calling > out that #VMGEXIT can be executed from userspace, and that before returning to > uesrspace the guest kernel must always ensure that the GCHB points at a legal > GPA _and_ all primary fields are marked invalid. Yes, the spec can be updated to include a "best practices" section for OSes and Hypervisors to follow without actually having to update the version of the GHCB spec, so that should be doable. Thanks, Tom >
