KVM: SVM: Check EFER.SVME and CPL on #GP intercept of SVM instructions

When KVM intercepts #GP on an SVM instruction from L2, it checks the
legality of RAX, and injects a #GP if RAX is illegal, or otherwise
synthesizes a #VMEXIT to L1. However, checking EFER.SVME and CPL takes
precedence over both the RAX check and the intercept. Call
nested_svm_check_permissions() first to cover both.

Note that if #GP is intercepted on SVM instruction in L1, the intercept
handlers of VMRUN/VMLOAD/VMSAVE already perform these checks.

Note #2, if KVM does not intercept #GP, the check for EFER.SVME is not
done in the correct order, because KVM handles it by intercepting the
instructions when EFER.SVME=0 and injecting #UD.  However, a #GP
injected by hardware would happen before the instruction intercept,
leading to #GP taking precedence over #UD from the guest's perspective.
Opportunistically add a FIXME for this.

Fixes: 82a11e9c6fa2 ("KVM: SVM: Add emulation support for #GP triggered by SVM instructions")
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260316202732.3164936-6-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 3472916657e15fce0f10d3bf3a1ba7c8298a3df2..7d0d95f40cd25952e754cf5da353535845ee6c37 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1054,6 +1054,11 @@ static void svm_recalc_instruction_intercepts(struct kvm_vcpu *vcpu)
         * No need to toggle any of the vgif/vls/etc. enable bits here, as they
         * are set when the VMCB is initialized and never cleared (if the
         * relevant intercepts are set, the enablements are meaningless anyway).
+        *
+        * FIXME: When #GP is not intercepted, a #GP on these instructions (e.g.
+        * due to CPL > 0) could be injected by hardware before the instruction
+        * is intercepted, leading to #GP taking precedence over #UD from the
+        * guest's perspective.
         */
        if (!(vcpu->arch.efer & EFER_SVME)) {
                svm_set_intercept(svm, INTERCEPT_VMLOAD);
@@ -2294,6 +2299,9 @@ static int gp_interception(struct kvm_vcpu *vcpu)
                if (!is_guest_mode(vcpu))
                        return svm_invoke_exit_handler(vcpu, svm_exit_code);
 
+               if (nested_svm_check_permissions(vcpu))
+                       return 1;
+
                if (!page_address_valid(vcpu, kvm_register_read(vcpu, VCPU_REGS_RAX)))
                        goto reinject;