Re: [PATCH] KVM: SEV: Return INVALID_INPUT on SNP req/resp buffer access failure

[Jacky Li]

On Wed, Sep 16, 2026 at 11:34 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> The problem is, I don't see how the guest can know when it needs to discard the
> VMPCK (post-request failures) versus when the VMPCK is still "fine" (pre-request
> errors).
>
> FWIW, I'm not terribly concerned about bleeding host issues into the guest, I'm
> more concerned about ending up with deferred fatalities and a mess of an "ABI"
> between the guest and KVM with respect to handling failures.
>
> What if we "tickle" the resp_gpa before doing the request?  Similar to how CPUs
> probe bytes early in XSAVE/XRSTOR to avoid having to unwind later on.  In this
> case, the response is restricted to a single page, so we only need to tickle a
> single byte.  E.g.
>

Agree. I see your points and why this tickle idea is a great solution.
Actually, building on that idea, what do you think about using
kvm_vcpu_map() in the exact place that you did
kvm_write_guest(kvm, resp_gpa, &tickle, sizeof(tickle)) instead (and
unmapping afterwards)? A few reasons:

1. Like kvm_write_guest(), kvm_vcpu_map() also checks the existence and
   writability of the memslot via __gfn_to_hva_many(..., write=true) and
   faults in the host page as writable, plus it pins the page across the
   firmware command.

2. kvm_vcpu_map() has cleaner semantics of acquiring and holding the
   guest response page in KVM until the response is delivered, rather
   than doing a dummy write to test the waters. This is somewhat
   consistent with the GHCB page itself, which stays mapped until the
   response is filled in.

3. Pinning the page prevents a malicious guest from converting resp_gpa
   to private between the pre-check and kvm_write_guest() after the
   guest_memfd in-place conversion patch series [1] is merged. (This
   assumes a future guest_memfd implementation, so it's less convincing
   than the first two points. This was also my original TOCTOU concern.
   And once we have either pre-check, on upstream today the guest has
   no way to change the memslot state before kvm_write_guest(), so I'll
   also drop the original TOCTOU commit message.)

[1] https://lore.kernel.org/lkml/20260830-gmem-inplace-conversion-v12-15-85e5fd25252a@xxxxxxxxxx/