[PATCH 1/3] KVM: x86: Emulator flag for instruction with no big real mode

[Nadav Amit]

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@xxxxxxxxxxxxxxxxx>
---
 arch/x86/kvm/emulate.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index e4e833d..f90194d 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -162,6 +162,7 @@
 #define NoWrite     ((u64)1 << 45)  /* No writeback */
 #define SrcWrite    ((u64)1 << 46)  /* Write back src operand */
 #define NoMod	    ((u64)1 << 47)  /* Mod field is ignored */
+#define NoBigReal   ((u64)1 << 48)  /* No big real mode */
 
 #define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
 
@@ -651,7 +652,12 @@ 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;
-- 
1.9.1

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