[bug report] RDMA/iw_cxgb4: Avoid touch after free error in ARP failure handlers

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Raju Rangoju,

The patch 1dad0ebeea1c: "RDMA/iw_cxgb4: Avoid touch after free error
in ARP failure handlers" from May 15, 2017, leads to the following
static checker warning:

	drivers/infiniband/hw/cxgb4/cm.c:4310 process_work()
	warn: 'skb' was already freed.

drivers/infiniband/hw/cxgb4/cm.c
  4289  static void process_work(struct work_struct *work)
  4290  {
  4291          struct sk_buff *skb = NULL;
  4292          struct c4iw_dev *dev;
  4293          struct cpl_act_establish *rpl;
  4294          unsigned int opcode;
  4295          int ret;
  4296  
  4297          process_timedout_eps();
  4298          while ((skb = skb_dequeue(&rxq))) {
  4299                  rpl = cplhdr(skb);
  4300                  dev = *((struct c4iw_dev **) (skb->cb + sizeof(void *)));
  4301                  opcode = rpl->ot.opcode;
  4302  
  4303                  if (opcode >= ARRAY_SIZE(work_handlers) ||
  4304                      !work_handlers[opcode]) {
  4305                          pr_err("No handler for opcode 0x%x.\n", opcode);
  4306                          kfree_skb(skb);
  4307                  } else {
  4308                          ret = work_handlers[opcode](dev, skb);
  4309                          if (!ret)
  4310                                  kfree_skb(skb);

I'm not sure why this warning didn't show up before... :(

We added some kfree_skb() calls to _put_ep_safe() and _put_pass_ep_safe().
The thing about kfree_skb() is that it's refcounted so it might not
free anything so this could be a false positive.  I've looked at the
code and it looks like it could be a bug?

  4311                  }
  4312                  process_timedout_eps();
  4313          }
  4314  }

regards,
dan carpenter