KVM: x86: clear vcpu->run->hypercall.ret before exiting for KVM_EXIT_HYPERCALL

QEMU up to 9.2.0 is assuming that vcpu->run->hypercall.ret is 0 on exit and
it never modifies it when processing KVM_EXIT_HYPERCALL.  Make this explicit
in the code, to avoid breakage when KVM starts modifying that field.

This in principle is not a good idea... It would have been much better if
KVM had set the field to -KVM_ENOSYS from the beginning, so that a dumb
userspace that does nothing on KVM_EXIT_HYPERCALL would tell the guest it
does not support KVM_HC_MAP_GPA_RANGE.  However, breaking userspace is
a Very Bad Thing, as everybody should know.

Reported-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: bonzini

arch/x86/kvm/svm/sev.c

@@ -3634,6 +3634,13 @@ static int snp_begin_psc_msr(struct vcpu_svm *svm, u64 ghcb_msr)
 
 	vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
 	vcpu->run->hypercall.nr = KVM_HC_MAP_GPA_RANGE;
+	/*
+	 * In principle this should have been -KVM_ENOSYS, but userspace (QEMU <=9.2)
+	 * assumed that vcpu->run->hypercall.ret is never changed by KVM and thus that
+	 * it was always zero on KVM_EXIT_HYPERCALL.  Since KVM is now overwriting
+	 * vcpu->run->hypercall.ret, ensuring that it is zero to not break QEMU.
+	 */
+	vcpu->run->hypercall.ret = 0;
 	vcpu->run->hypercall.args[0] = gpa;
 	vcpu->run->hypercall.args[1] = 1;
 	vcpu->run->hypercall.args[2] = (op == SNP_PAGE_STATE_PRIVATE)
@@ -3797,6 +3804,13 @@ static int snp_begin_psc(struct vcpu_svm *svm, struct psc_buffer *psc)
 	case VMGEXIT_PSC_OP_SHARED:
 		vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
 		vcpu->run->hypercall.nr = KVM_HC_MAP_GPA_RANGE;
+		/*
+		 * In principle this should have been -KVM_ENOSYS, but userspace (QEMU <=9.2)
+		 * assumed that vcpu->run->hypercall.ret is never changed by KVM and thus that
+		 * it was always zero on KVM_EXIT_HYPERCALL.  Since KVM is now overwriting
+		 * vcpu->run->hypercall.ret, ensuring that it is zero to not break QEMU.
+		 */
+		vcpu->run->hypercall.ret = 0;
 		vcpu->run->hypercall.args[0] = gfn_to_gpa(gfn);
 		vcpu->run->hypercall.args[1] = npages;
 		vcpu->run->hypercall.args[2] = entry_start.operation == VMGEXIT_PSC_OP_PRIVATE

arch/x86/kvm/x86.c

@@ -10052,6 +10052,13 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
 
 		vcpu->run->exit_reason        = KVM_EXIT_HYPERCALL;
 		vcpu->run->hypercall.nr       = KVM_HC_MAP_GPA_RANGE;
+		/*
+		 * In principle this should have been -KVM_ENOSYS, but userspace (QEMU <=9.2)
+		 * assumed that vcpu->run->hypercall.ret is never changed by KVM and thus that
+		 * it was always zero on KVM_EXIT_HYPERCALL.  Since KVM is now overwriting
+		 * vcpu->run->hypercall.ret, ensuring that it is zero to not break QEMU.
+		 */
+		vcpu->run->hypercall.ret = 0;
 		vcpu->run->hypercall.args[0]  = gpa;
 		vcpu->run->hypercall.args[1]  = npages;
 		vcpu->run->hypercall.args[2]  = attrs;