KVM: Fix SS default ESP/EBP based addressing

We correctly default to SS when BP is used as a base in 16-bit address mode,
but we don't do that for 32-bit mode.

Fix by adjusting the default to SS when either ESP or EBP is used as the base
register.

Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 33ccd757cb11..7552c0ac6e76 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -974,6 +974,12 @@ static void decode_register_operand(struct x86_emulate_ctxt *ctxt,
         op->orig_val = op->val;
 }
 
+static void adjust_modrm_seg(struct x86_emulate_ctxt *ctxt, int base_reg)
+{
+        if (base_reg == VCPU_REGS_RSP || base_reg == VCPU_REGS_RBP)
+                ctxt->modrm_seg = VCPU_SREG_SS;
+}
+
 static int decode_modrm(struct x86_emulate_ctxt *ctxt,
                         struct operand *op)
 {
@@ -1077,15 +1083,20 @@ static int decode_modrm(struct x86_emulate_ctxt *ctxt,
 
                         if ((base_reg & 7) == 5 && ctxt->modrm_mod == 0)
                                 modrm_ea += insn_fetch(s32, ctxt);
-                        else
+                        else {
                                 modrm_ea += ctxt->regs[base_reg];
+                                adjust_modrm_seg(ctxt, base_reg);
+                        }
                         if (index_reg != 4)
                                 modrm_ea += ctxt->regs[index_reg] << scale;
                 } else if ((ctxt->modrm_rm & 7) == 5 && ctxt->modrm_mod == 0) {
                         if (ctxt->mode == X86EMUL_MODE_PROT64)
                                 ctxt->rip_relative = 1;
-                } else
-                        modrm_ea += ctxt->regs[ctxt->modrm_rm];
+                } else {
+                        base_reg = ctxt->modrm_rm;
+                        modrm_ea += ctxt->regs[base_reg];
+                        adjust_modrm_seg(ctxt, base_reg);
+                }
                 switch (ctxt->modrm_mod) {
                 case 0:
                         if (ctxt->modrm_rm == 5)