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

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 10, 2026, Jacky Li wrote:
> Currently, snp_handle_(ext_)guest_req() returns -EIO when
> kvm_{read/write/clear}_guest() fails while accessing guest-provided
> buffers. Returning -EIO causes KVM_RUN to exit to userspace, likely
> killing the VM.
> 
> Fix this by returning GHCB_HV_RESP_MALFORMED_INPUT with sub-error code
> GHCB_ERR_INVALID_INPUT to the guest and resuming the vCPU.
> 
> Per the GHCB specification, guest-provided GPA buffers that cannot
> be accessed by the hypervisor (e.g. private pages) should be treated
> as guest input errors. Because kvm_{read/write/clear}_guest() only
> returns -EFAULT on failure, treating this failure as an invalid input
> aligns with the definition of -EFAULT ("Bad address").
> 
> Returning GHCB_ERR_INVALID_INPUT also matches existing SNP handling
> in KVM, which already returns this error code for unaligned or
> overlapping buffers. It also aligns with other hypercall implementations
> in KVM (e.g. Hyper-V returning INVALID_HYPERCALL_INPUT on
> kvm_read_guest() failures in kvm_hv_flush_tlb()).
> 
> Performing upfront validation (e.g. via kvm_mem_is_private()) is
> avoided because it is prone to TOCTOU races with concurrent Page State
> Changes.

When stating what a patch does (or doesn't) do, phrase everything as commands.
Passively describing the patch, as done above, is problematic as it's not clear
if the changelog is talking about what the patch itself is (not) doing, or if
it's talking about the side effects of the changes.  Whereas this:

  Don't try to validate guest-provide ahead of time, as such checks are prone
  to TOCTOU races, e.g. with Page State Changes, memslot updates, etc.

is more obviously talking about the patch.

> Fixes: 88caf544c930 ("KVM: SEV: Provide support for SNP_GUEST_REQUEST NAE event")
> Fixes: 74458e4859d8 ("KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event")
> Signed-off-by: Jacky Li <jackyli@google.com>
> ---
>  arch/x86/kvm/svm/sev.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 5705723f1f41..d07562310519 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -4228,8 +4228,10 @@ static int snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_
>  
>  	guard(mutex)(&sev->guest_req_mutex);
>  
> -	if (kvm_read_guest(kvm, req_gpa, sev->guest_req_buf, PAGE_SIZE))
> -		return -EIO;
> +	if (kvm_read_guest(kvm, req_gpa, sev->guest_req_buf, PAGE_SIZE)) {

I don't love that userspace VMM goofs will bleed into the guest, but on the other
hand, KVM already uses this pattern for Hyper-V hypercalls (and worse patterns
for KVM-defined PV features), and practically speaking this is better behavior
than returning -EIO.  So I'm good with this.