KVM: x86: Allow userspace to opt out of hypercall patching

KVM handles the VMCALL/VMMCALL instructions very strangely. Even though
both of these instructions really should #UD when executed on the wrong
vendor's hardware (i.e. VMCALL on SVM, VMMCALL on VMX), KVM replaces the
guest's instruction with the appropriate instruction for the vendor.
Nonetheless, older guest kernels without commit c1118b36 ("x86: kvm:
use alternatives for VMCALL vs. VMMCALL if kernel text is read-only")
do not patch in the appropriate instruction using alternatives, likely
motivating KVM's intervention.

Add a quirk allowing userspace to opt out of hypercall patching. If the
quirk is disabled, KVM synthesizes a #UD in the guest.
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20220316005538.2282772-2-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -7134,6 +7134,15 @@ The valid bits in cap.args[0] are:
                                     Additionally, when this quirk is disabled,
                                     KVM clears CPUID.01H:ECX[bit 3] if
                                     IA32_MISC_ENABLE[bit 18] is cleared.
+
+ KVM_X86_QUIRK_FIX_HYPERCALL_INSN   By default, KVM rewrites guest
+                                    VMMCALL/VMCALL instructions to match the
+                                    vendor's hypercall instruction for the
+                                    system. When this quirk is disabled, KVM
+                                    will no longer rewrite invalid guest
+                                    hypercall instructions. Executing the
+                                    incorrect hypercall instruction will
+                                    generate a #UD within the guest.
 =================================== ============================================
 
 8. Other capabilities.

arch/x86/include/asm/kvm_host.h

@@ -1987,6 +1987,7 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
 	 KVM_X86_QUIRK_CD_NW_CLEARED |		\
 	 KVM_X86_QUIRK_LAPIC_MMIO_HOLE |	\
 	 KVM_X86_QUIRK_OUT_7E_INC_RIP |		\
-	 KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)
+	 KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT |	\
+	 KVM_X86_QUIRK_FIX_HYPERCALL_INSN)
 
 #endif /* _ASM_X86_KVM_HOST_H */

arch/x86/include/uapi/asm/kvm.h

@@ -428,11 +428,12 @@ struct kvm_sync_regs {
 	struct kvm_vcpu_events events;
 };
 
-#define KVM_X86_QUIRK_LINT0_REENABLED	   (1 << 0)
-#define KVM_X86_QUIRK_CD_NW_CLEARED	   (1 << 1)
-#define KVM_X86_QUIRK_LAPIC_MMIO_HOLE	   (1 << 2)
-#define KVM_X86_QUIRK_OUT_7E_INC_RIP	   (1 << 3)
-#define KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT (1 << 4)
+#define KVM_X86_QUIRK_LINT0_REENABLED		(1 << 0)
+#define KVM_X86_QUIRK_CD_NW_CLEARED		(1 << 1)
+#define KVM_X86_QUIRK_LAPIC_MMIO_HOLE		(1 << 2)
+#define KVM_X86_QUIRK_OUT_7E_INC_RIP		(1 << 3)
+#define KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT	(1 << 4)
+#define KVM_X86_QUIRK_FIX_HYPERCALL_INSN	(1 << 5)
 
 #define KVM_STATE_NESTED_FORMAT_VMX	0
 #define KVM_STATE_NESTED_FORMAT_SVM	1

arch/x86/kvm/x86.c

@@ -9267,6 +9267,17 @@ static int emulator_fix_hypercall(struct x86_emulate_ctxt *ctxt)
 	char instruction[3];
 	unsigned long rip = kvm_rip_read(vcpu);
 
+	/*
+	 * If the quirk is disabled, synthesize a #UD and let the guest pick up
+	 * the pieces.
+	 */
+	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_FIX_HYPERCALL_INSN)) {
+		ctxt->exception.error_code_valid = false;
+		ctxt->exception.vector = UD_VECTOR;
+		ctxt->have_exception = true;
+		return X86EMUL_PROPAGATE_FAULT;
+	}
+
 	static_call(kvm_x86_patch_hypercall)(vcpu, instruction);
 
 	return emulator_write_emulated(ctxt, rip, instruction, 3,