ipv4: prevent speculative execution

CVE-2017-5753 (Spectre v1 Intel)

Since the offset value in function raw_getfrag()
seems to be controllable by userspace and later on
conditionally (upon bound check) used in the following
memcpy, insert an observable speculation
barrier before its usage. This should prevent
observable speculation on that branch and avoid
kernel memory leak.
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/ipv4/raw.c

@@ -466,6 +466,7 @@ static int raw_getfrag(void *from, char *to, int offset, int len, int odd,
 	if (offset < rfv->hlen) {
 		int copy = min(rfv->hlen - offset, len);
 
+		osb();
 		if (skb->ip_summed == CHECKSUM_PARTIAL)
 			memcpy(to, rfv->hdr.c + offset, copy);
 		else