[PATCH 2/3] KVM: x86/xen: Only do in-kernel acceleration of hypercalls for guest CPL0

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From: David Woodhouse <dwmw@xxxxxxxxxxxx>

There are almost no hypercalls which are valid from CPL > 0, and definitely
none which are handled by the kernel.

Fixes: 2fd6df2f2b47 ("KVM: x86/xen: intercept EVTCHNOP_send from guests")
Reported-by: Michal Luczaj <mhal@xxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxx
---
 arch/x86/kvm/xen.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index dc2f304f2e69..f3098c0e386a 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1227,6 +1227,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 	bool longmode;
 	u64 input, params[6], r = -ENOSYS;
 	bool handled = false;
+	u8 cpl;
 
 	input = (u64)kvm_register_read(vcpu, VCPU_REGS_RAX);
 
@@ -1254,9 +1255,17 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 		params[5] = (u64)kvm_r9_read(vcpu);
 	}
 #endif
+	cpl = static_call(kvm_x86_get_cpl)(vcpu);
 	trace_kvm_xen_hypercall(input, params[0], params[1], params[2],
 				params[3], params[4], params[5]);
 
+	/*
+	 * Only allow hypercall acceleration for CPL0. The rare hypercalls that
+	 * are permitted in guest userspace can be handled by the VMM.
+	 */
+	if (unlikely(cpl > 0))
+		goto handle_in_userspace;
+
 	switch (input) {
 	case __HYPERVISOR_xen_version:
 		if (params[0] == XENVER_version && vcpu->kvm->arch.xen.xen_version) {
@@ -1291,10 +1300,11 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 	if (handled)
 		return kvm_xen_hypercall_set_result(vcpu, r);
 
+handle_in_userspace:
 	vcpu->run->exit_reason = KVM_EXIT_XEN;
 	vcpu->run->xen.type = KVM_EXIT_XEN_HCALL;
 	vcpu->run->xen.u.hcall.longmode = longmode;
-	vcpu->run->xen.u.hcall.cpl = static_call(kvm_x86_get_cpl)(vcpu);
+	vcpu->run->xen.u.hcall.cpl = cpl;
 	vcpu->run->xen.u.hcall.input = input;
 	vcpu->run->xen.u.hcall.params[0] = params[0];
 	vcpu->run->xen.u.hcall.params[1] = params[1];
-- 
2.35.3




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