KVM: x86 emulator: Add sysexit emulation

Handle #UD intercept of the sysexit instruction in 64bit mode returning to
32bit compat mode on an AMD host.
Setup the segment descriptors for CS and SS and the EIP/ESP registers
according to the manual.

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, 71 insertions, 1 deletions

diff --git a/arch/x86/kvm/x86_emulate.c b/arch/x86/kvm/x86_emulate.c
index 7a9bddb3ebd..c6663d46f32 100644
--- a/arch/x86/kvm/x86_emulate.c
+++ b/arch/x86/kvm/x86_emulate.c
@@ -1541,6 +1541,73 @@ emulate_sysenter(struct x86_emulate_ctxt *ctxt)
         return 0;
 }
 
+static int
+emulate_sysexit(struct x86_emulate_ctxt *ctxt)
+{
+        struct decode_cache *c = &ctxt->decode;
+        struct kvm_segment cs, ss;
+        u64 msr_data;
+        int usermode;
+
+        /* inject #UD if LOCK prefix is used */
+        if (c->lock_prefix)
+                return -1;
+
+        /* inject #GP if in real mode or paging is disabled */
+        if (ctxt->mode == X86EMUL_MODE_REAL
+                || !(ctxt->vcpu->arch.cr0 & X86_CR0_PE)) {
+                kvm_inject_gp(ctxt->vcpu, 0);
+                return -1;
+        }
+
+        /* sysexit must be called from CPL 0 */
+        if (kvm_x86_ops->get_cpl(ctxt->vcpu) != 0) {
+                kvm_inject_gp(ctxt->vcpu, 0);
+                return -1;
+        }
+
+        setup_syscalls_segments(ctxt, &cs, &ss);
+
+        if ((c->rex_prefix & 0x8) != 0x0)
+                usermode = X86EMUL_MODE_PROT64;
+        else
+                usermode = X86EMUL_MODE_PROT32;
+
+        cs.dpl = 3;
+        ss.dpl = 3;
+        kvm_x86_ops->get_msr(ctxt->vcpu, MSR_IA32_SYSENTER_CS, &msr_data);
+        switch (usermode) {
+        case X86EMUL_MODE_PROT32:
+                cs.selector = (u16)(msr_data + 16);
+                if ((msr_data & 0xfffc) == 0x0) {
+                        kvm_inject_gp(ctxt->vcpu, 0);
+                        return -1;
+                }
+                ss.selector = (u16)(msr_data + 24);
+                break;
+        case X86EMUL_MODE_PROT64:
+                cs.selector = (u16)(msr_data + 32);
+                if (msr_data == 0x0) {
+                        kvm_inject_gp(ctxt->vcpu, 0);
+                        return -1;
+                }
+                ss.selector = cs.selector + 8;
+                cs.db = 0;
+                cs.l = 1;
+                break;
+        }
+        cs.selector |= SELECTOR_RPL_MASK;
+        ss.selector |= SELECTOR_RPL_MASK;
+
+        kvm_x86_ops->set_segment(ctxt->vcpu, &cs, VCPU_SREG_CS);
+        kvm_x86_ops->set_segment(ctxt->vcpu, &ss, VCPU_SREG_SS);
+
+        c->eip = ctxt->vcpu->arch.regs[VCPU_REGS_RDX];
+        c->regs[VCPU_REGS_RSP] = ctxt->vcpu->arch.regs[VCPU_REGS_RCX];
+
+        return 0;
+}
+
 int
 x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 {
@@ -2215,7 +2282,10 @@ twobyte_insn:
                         goto writeback;
                 break;
         case 0x35:              /* sysexit */
-                goto cannot_emulate;
+                if (emulate_sysexit(ctxt) == -1)
+                        goto cannot_emulate;
+                else
+                        goto writeback;
                 break;
         case 0x40 ... 0x4f:     /* cmov */
                 c->dst.val = c->dst.orig_val = c->src.val;