[stable-6.12 3/3] KVM: SEV: Ignore Port I/O requests of length '0'

[Jack Wang <jinpu.wang@ionos.com>]

From: Sean Christopherson <seanjc@google.com>

commit 3988bd2723de407ae90fa7a6f6029b4e60238c58 upstream.

Explicitly ignore Port I/O requests of length '0' (or count '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 WARNing on trying
to configure the scratch area with len==0.

Fixes: 291bd20d5d88 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")
Cc: stable@vger.kernel.org
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260501202250.2115252-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
---
 arch/x86/kvm/svm/sev.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 82b26899b9a4..7e8b5acc2133 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4451,6 +4451,11 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 			    control->exit_info_1, control->exit_info_2);
 		ret = -EINVAL;
 		break;
+	case SVM_EXIT_IOIO:
+		if (!((control->exit_info_1 & SVM_IOIO_SIZE_MASK) >> SVM_IOIO_SIZE_SHIFT))
+			return 1;
+
+		fallthrough;
 	default:
 		ret = svm_invoke_exit_handler(vcpu, exit_code);
 	}
@@ -4471,6 +4476,9 @@ int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in)
 	if (unlikely(check_mul_overflow(count, size, &bytes)))
 		return -EINVAL;
 
+	if (!bytes)
+		return 1;
+
 	r = setup_vmgexit_scratch(svm, in, bytes);
 	if (r)
 		return r;
-- 
2.43.0