KVM: X86: Fix operand/address-size during instruction decoding

Pedro reported:
  During tests that we conducted on KVM, we noticed that executing a "PUSH %ES"
  instruction under KVM produces different results on both memory and the SP
  register depending on whether EPT support is enabled. With EPT the SP is
  reduced by 4 bytes (and the written value is 0-padded) but without EPT support
  it is only reduced by 2 bytes. The difference can be observed when the CS.DB
  field is 1 (32-bit) but not when it's 0 (16-bit).

The internal segment descriptor cache exist even in real/vm8096 mode. The CS.D
also should be respected instead of just default operand/address-size/66H
prefix/67H prefix during instruction decoding. This patch fixes it by also
adjusting operand/address-size according to CS.D.

Reported-by: Pedro Fonseca <pfonseca@cs.washington.edu>
Tested-by: Pedro Fonseca <pfonseca@cs.washington.edu>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Pedro Fonseca <pfonseca@cs.washington.edu>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 8079d141792af91994421d15c19c26d3bd386c59..b4a87debbb4b3b22c1664c7aaa1947cc4b630d85 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5000,6 +5000,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
        bool op_prefix = false;
        bool has_seg_override = false;
        struct opcode opcode;
+       u16 dummy;
+       struct desc_struct desc;
 
        ctxt->memop.type = OP_NONE;
        ctxt->memopp = NULL;
@@ -5018,6 +5020,11 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
        switch (mode) {
        case X86EMUL_MODE_REAL:
        case X86EMUL_MODE_VM86:
+               def_op_bytes = def_ad_bytes = 2;
+               ctxt->ops->get_segment(ctxt, &dummy, &desc, NULL, VCPU_SREG_CS);
+               if (desc.d)
+                       def_op_bytes = def_ad_bytes = 4;
+               break;
        case X86EMUL_MODE_PROT16:
                def_op_bytes = def_ad_bytes = 2;
                break;