[PATCH 3.17 126/319] KVM: x86: Decoding guest instructions which cross page boundary may fail

[Greg Kroah-Hartman]

3.17-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>

commit 08da44aedba0f493e10695fa334348a7a4f72eb3 upstream.

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.

Fixes: 5cfc7e0f5e5e1adf998df94f8e36edaf5d30d38e
Signed-off-by: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/kvm/emulate.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -773,8 +773,10 @@ static int __do_insn_fetch_bytes(struct
 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;
 }


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