KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

There is no easy way to force KVM to run an instruction through the emulator
(by design as that will expose the x86 emulator as a significant attack-surface).
However, we do wish to expose the x86 emulator in case we are testing it
(e.g. via kvm-unit-tests). Therefore, this patch adds a "force emulation prefix"
that is designed to raise #UD which KVM will trap and it's #UD exit-handler will
match "force emulation prefix" to run instruction after prefix by the x86 emulator.
To not expose the x86 emulator by default, we add a module parameter that should
be off by default.

A simple testcase here:

    #include <stdio.h>
    #include <string.h>

    #define HYPERVISOR_INFO 0x40000000

    #define CPUID(idx, eax, ebx, ecx, edx) \
        asm volatile (\
        "ud2a; .ascii \"kvm\"; cpuid" \
        :"=b" (*ebx), "=a" (*eax), "=c" (*ecx), "=d" (*edx) \
            :"0"(idx) );

    void main()
    {
        unsigned int eax, ebx, ecx, edx;
        char string[13];

        CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx);
        *(unsigned int *)(string + 0) = ebx;
        *(unsigned int *)(string + 4) = ecx;
        *(unsigned int *)(string + 8) = edx;

        string[12] = 0;
        if (strncmp(string, "KVMKVMKVM\0\0\0", 12) == 0)
            printf("kvm guest\n");
        else
            printf("bare hardware\n");
    }

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
[Correctly handle usermode exits. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -146,6 +146,9 @@ bool __read_mostly enable_vmware_backdoor = false;
 module_param(enable_vmware_backdoor, bool, S_IRUGO);
 EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
 
+static bool __read_mostly force_emulation_prefix = false;
+module_param(force_emulation_prefix, bool, S_IRUGO);
+
 #define KVM_NR_SHARED_MSRS 16
 
 struct kvm_shared_msrs_global {
@@ -4842,9 +4845,20 @@ EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
 
 int handle_ud(struct kvm_vcpu *vcpu)
 {
+	int emul_type = EMULTYPE_TRAP_UD;
 	enum emulation_result er;
+	char sig[5]; /* ud2; .ascii "kvm" */
+	struct x86_exception e;
+
+	if (force_emulation_prefix &&
+	    kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
+				kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e) == 0 &&
+	    memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
+		kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
+		emul_type = 0;
+	}
 
-	er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
+	er = emulate_instruction(vcpu, emul_type);
 	if (er == EMULATE_USER_EXIT)
 		return 0;
 	if (er != EMULATE_DONE)