KVM: x86: Emulation of call may use incorrect stack size

On long-mode, when far call that changes cs.l takes place, the stack size is
determined by the new mode.  For instance, if we go from 32-bit mode to 64-bit
mode, the stack-size if 64.  KVM uses the old stack size.

Fix it.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 18 insertions, 7 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index c3b07574942f..81dcf7964701 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -741,19 +741,26 @@ static int assign_eip_far(struct x86_emulate_ctxt *ctxt, ulong dst,
                           const struct desc_struct *cs_desc)
 {
         enum x86emul_mode mode = ctxt->mode;
+        int rc;
 
 #ifdef CONFIG_X86_64
-        if (ctxt->mode >= X86EMUL_MODE_PROT32 && cs_desc->l) {
-                u64 efer = 0;
+        if (ctxt->mode >= X86EMUL_MODE_PROT16) {
+                if (cs_desc->l) {
+                        u64 efer = 0;
 
-                ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
-                if (efer & EFER_LMA)
-                        mode = X86EMUL_MODE_PROT64;
+                        ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
+                        if (efer & EFER_LMA)
+                                mode = X86EMUL_MODE_PROT64;
+                } else
+                        mode = X86EMUL_MODE_PROT32; /* temporary value */
         }
 #endif
         if (mode == X86EMUL_MODE_PROT16 || mode == X86EMUL_MODE_PROT32)
                 mode = cs_desc->d ? X86EMUL_MODE_PROT32 : X86EMUL_MODE_PROT16;
-        return assign_eip(ctxt, dst, mode);
+        rc = assign_eip(ctxt, dst, mode);
+        if (rc == X86EMUL_CONTINUE)
+                ctxt->mode = mode;
+        return rc;
 }
 
 static inline int jmp_rel(struct x86_emulate_ctxt *ctxt, int rel)
@@ -3062,6 +3069,7 @@ static int em_call_far(struct x86_emulate_ctxt *ctxt)
         struct desc_struct old_desc, new_desc;
         const struct x86_emulate_ops *ops = ctxt->ops;
         int cpl = ctxt->ops->cpl(ctxt);
+        enum x86emul_mode prev_mode = ctxt->mode;
 
         old_eip = ctxt->_eip;
         ops->get_segment(ctxt, &old_cs, &old_desc, NULL, VCPU_SREG_CS);
@@ -3085,11 +3093,14 @@ static int em_call_far(struct x86_emulate_ctxt *ctxt)
         rc = em_push(ctxt);
         /* If we failed, we tainted the memory, but the very least we should
            restore cs */
-        if (rc != X86EMUL_CONTINUE)
+        if (rc != X86EMUL_CONTINUE) {
+                pr_warn_once("faulting far call emulation tainted memory\n");
                 goto fail;
+        }
         return rc;
 fail:
         ops->set_segment(ctxt, old_cs, &old_desc, 0, VCPU_SREG_CS);
+        ctxt->mode = prev_mode;
         return rc;
 
 }