KVM: Allow emulation of syscalls instructions on #UD

Add the opcodes for syscall, sysenter and sysexit to the list of instructions
handled by the undefined opcode handler.

Signed-off-by: Christoph Egger <christoph.egger@amd.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 26 insertions, 7 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 892a7a60c815..57e76b37242f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2667,14 +2667,33 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 
                 r = x86_decode_insn(&vcpu->arch.emulate_ctxt, &emulate_ops);
 
-                /* Reject the instructions other than VMCALL/VMMCALL when
-                 * try to emulate invalid opcode */
+                /* Only allow emulation of specific instructions on #UD
+                 * (namely VMMCALL, sysenter, sysexit, syscall)*/
                 c = &vcpu->arch.emulate_ctxt.decode;
-                if ((emulation_type & EMULTYPE_TRAP_UD) &&
-                    (!(c->twobyte && c->b == 0x01 &&
-                      (c->modrm_reg == 0 || c->modrm_reg == 3) &&
-                       c->modrm_mod == 3 && c->modrm_rm == 1)))
-                        return EMULATE_FAIL;
+                if (emulation_type & EMULTYPE_TRAP_UD) {
+                        if (!c->twobyte)
+                                return EMULATE_FAIL;
+                        switch (c->b) {
+                        case 0x01: /* VMMCALL */
+                                if (c->modrm_mod != 3 || c->modrm_rm != 1)
+                                        return EMULATE_FAIL;
+                                break;
+                        case 0x34: /* sysenter */
+                        case 0x35: /* sysexit */
+                                if (c->modrm_mod != 0 || c->modrm_rm != 0)
+                                        return EMULATE_FAIL;
+                                break;
+                        case 0x05: /* syscall */
+                                if (c->modrm_mod != 0 || c->modrm_rm != 0)
+                                        return EMULATE_FAIL;
+                                break;
+                        default:
+                                return EMULATE_FAIL;
+                        }
+
+                        if (!(c->modrm_reg == 0 || c->modrm_reg == 3))
+                                return EMULATE_FAIL;
+                }
 
                 ++vcpu->stat.insn_emulation;
                 if (r)  {