[PATCH 02/24] KVM: SEV: Ignore MMIO requests of length '0'

[Paolo Bonzini]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

Explicitly ignore MMIO requests of length '0', so that setting up the
software scratch area (and other code) doesn't have to worry about
underflowing the length, and to allow for special casing '0' in the
future.

Fixes: 8f423a80d299 ("KVM: SVM: Support MMIO for an SEV-ES guest")
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Message-ID: <20260501202250.2115252-3-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/svm/sev.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 23170b64f4a3..fb2174b6d1ba 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4497,13 +4497,17 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 	case SVM_VMGEXIT_MMIO_READ:
 	case SVM_VMGEXIT_MMIO_WRITE: {
 		bool is_write = control->exit_code == SVM_VMGEXIT_MMIO_WRITE;
+		u64 len = control->exit_info_2;
 
-		ret = setup_vmgexit_scratch(svm, !is_write, control->exit_info_2);
+		if (!len)
+			return 1;
+
+		ret = setup_vmgexit_scratch(svm, !is_write, len);
 		if (ret)
 			break;
 
-		ret = kvm_sev_es_mmio(vcpu, is_write, control->exit_info_1,
-				      control->exit_info_2, svm->sev_es.ghcb_sa);
+		ret = kvm_sev_es_mmio(vcpu, is_write, control->exit_info_1, len,
+				      svm->sev_es.ghcb_sa);
 		break;
 	}
 	case SVM_VMGEXIT_NMI_COMPLETE:
-- 
2.54.0