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>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index c3b07574942fa3169ec6e9f5460ccfcc5dd9100a..81dcf7964701f04b423799622260456fc8fcadf8 100644 (file)
--- 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;
 
 }