KVM: x86: Decoding guest instructions which cross page boundary may fail

Once an instruction crosses a page boundary, the size read from the second page
disregards the common case that part of the operand resides on the first page.
As a result, fetch of long insturctions may fail, and thereby cause the
decoding to fail as well.

Cc: stable@vger.kernel.org
Fixes: 5cfc7e0
Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -778,8 +778,10 @@ static int __do_insn_fetch_bytes(struct x86_emulate_ctxt *ctxt, int op_size)
 static __always_inline int do_insn_fetch_bytes(struct x86_emulate_ctxt *ctxt,
 					       unsigned size)
 {
-	if (unlikely(ctxt->fetch.end - ctxt->fetch.ptr < size))
-		return __do_insn_fetch_bytes(ctxt, size);
+	unsigned done_size = ctxt->fetch.end - ctxt->fetch.ptr;
+
+	if (unlikely(done_size < size))
+		return __do_insn_fetch_bytes(ctxt, size - done_size);
 	else
 		return X86EMUL_CONTINUE;
 }