[PATCH 4.4 67/69] kvm: x86: Check memopp before dereference (CVE-2016-8630)

[Greg Kroah-Hartman]

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

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

From: Owen Hofmann <osh@xxxxxxxxxx>

commit d9092f52d7e61dd1557f2db2400ddb430e85937e upstream.

Commit 41061cdb98 ("KVM: emulate: do not initialize memopp") removes a
check for non-NULL under incorrect assumptions. An undefined instruction
with a ModR/M byte with Mod=0 and R/M-5 (e.g. 0xc7 0x15) will attempt
to dereference a null pointer here.

Fixes: 41061cdb98a0bec464278b4db8e894a3121671f5
Message-Id: <1477592752-126650-2-git-send-email-osh@xxxxxxxxxx>
Signed-off-by: Owen Hofmann <osh@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 arch/x86/kvm/emulate.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -5033,7 +5033,7 @@ done_prefixes:
 	/* Decode and fetch the destination operand: register or memory. */
 	rc = decode_operand(ctxt, &ctxt->dst, (ctxt->d >> DstShift) & OpMask);
 
-	if (ctxt->rip_relative)
+	if (ctxt->rip_relative && likely(ctxt->memopp))
 		ctxt->memopp->addr.mem.ea = address_mask(ctxt,
 					ctxt->memopp->addr.mem.ea + ctxt->_eip);