KVM: x86/xen: Only do in-kernel acceleration of hypercalls for guest CPL0

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@rbox.co>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Cc: stable@kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index dc2f304..f3098c0 100644 (file)
--- 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];