libfc: sanity check cpu number extracted from xid

In the receive path libfc extracts a cpu number from the ox_id in the
fiber channel header and uses that to do a per_cpu_ptr conversion.  If,
for some reason, a frame is received with an invalid ox_id, per_cpu_ptr
will return an invalid pointer and the libfc receive path will panic the
system trying to use it.

I'm currently looking at such a case, and I don't yet know why a cpu
number > nr_cpu_ids is appearing in an exchange id.  But adding a sanity
check in libfc prevents a system panic, and seems like good idea when
dealing with frames coming in from the network.
Signed-off-by: Chris Leech <cleech@redhat.com>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/libfc/fc_exch.c

@@ -908,9 +908,17 @@ static struct fc_exch *fc_exch_find(struct fc_exch_mgr *mp, u16 xid)
 {
 	struct fc_exch_pool *pool;
 	struct fc_exch *ep = NULL;
+	u16 cpu = xid & fc_cpu_mask;
+
+	if (cpu >= nr_cpu_ids || !cpu_possible(cpu)) {
+		printk_ratelimited(KERN_ERR
+			"libfc: lookup request for XID = %d, "
+			"indicates invalid CPU %d\n", xid, cpu);
+		return NULL;
+	}
 
 	if ((xid >= mp->min_xid) && (xid <= mp->max_xid)) {
-		pool = per_cpu_ptr(mp->pool, xid & fc_cpu_mask);
+		pool = per_cpu_ptr(mp->pool, cpu);
 		spin_lock_bh(&pool->lock);
 		ep = fc_exch_ptr_get(pool, (xid - mp->min_xid) >> fc_cpu_order);
 		if (ep) {