KVM: x86: Emulator flag for instruction that only support 16-bit addresses in real mode

Certain instructions, such as monitor and xsave do not support big real mode
and cause a #GP exception if any of the accessed bytes effective address are
not within [0, 0xffff].  This patch introduces a flag to mark these
instructions, including the necassary checks.

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

1 files changed, 7 insertions, 1 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 6a1d60956d63..b61ffe9d86a7 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -164,6 +164,7 @@
 #define NoMod       ((u64)1 << 47)  /* Mod field is ignored */
 #define Intercept   ((u64)1 << 48)  /* Has valid intercept field */
 #define CheckPerm   ((u64)1 << 49)  /* Has valid check_perm field */
+#define NoBigReal   ((u64)1 << 50)  /* No big real mode */
 
 #define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
 
@@ -640,7 +641,12 @@ static int __linearize(struct x86_emulate_ctxt *ctxt,
                 if (!fetch && (desc.type & 8) && !(desc.type & 2))
                         goto bad;
                 lim = desc_limit_scaled(&desc);
-                if ((desc.type & 8) || !(desc.type & 4)) {
+                if ((ctxt->mode == X86EMUL_MODE_REAL) && !fetch &&
+                    (ctxt->d & NoBigReal)) {
+                        /* la is between zero and 0xffff */
+                        if (la > 0xffff || (u32)(la + size - 1) > 0xffff)
+                                goto bad;
+                } else if ((desc.type & 8) || !(desc.type & 4)) {
                         /* expand-up segment */
                         if (addr.ea > lim || (u32)(addr.ea + size - 1) > lim)
                                 goto bad;