There is a race condition where ib_nl_make_request() inserts the request
data into the linked list but the timer in ib_nl_request_timeout() can see
it and destroy it before ib_nl_send_msg() is done touching it. This could
happen, for instance, if there is a long delay allocating memory during
nlmsg_new()

This causes a use-after-free in the send_mad() thread:

  [<ffffffffa02f43cb>] ? ib_pack+0x17b/0x240 [ib_core]
  [ <ffffffffa032aef1>] ib_sa_path_rec_get+0x181/0x200 [ib_sa]
  [<ffffffffa0379db0>] rdma_resolve_route+0x3c0/0x8d0 [rdma_cm]
  [<ffffffffa0374450>] ? cma_bind_port+0xa0/0xa0 [rdma_cm]
  [<ffffffffa040f850>] ? rds_rdma_cm_event_handler_cmn+0x850/0x850 [rds_rdma]
  [<ffffffffa040f22c>] rds_rdma_cm_event_handler_cmn+0x22c/0x850 [rds_rdma]
  [<ffffffffa040f860>] rds_rdma_cm_event_handler+0x10/0x20 [rds_rdma]
  [<ffffffffa037778e>] addr_handler+0x9e/0x140 [rdma_cm]
  [<ffffffffa026cdb4>] process_req+0x134/0x190 [ib_addr]
  [<ffffffff810a02f9>] process_one_work+0x169/0x4a0
  [<ffffffff810a0b2b>] worker_thread+0x5b/0x560
  [<ffffffff810a0ad0>] ? flush_delayed_work+0x50/0x50
  [<ffffffff810a68fb>] kthread+0xcb/0xf0
  [<ffffffff816ec49a>] ? __schedule+0x24a/0x810
  [<ffffffff816ec49a>] ? __schedule+0x24a/0x810
  [<ffffffff810a6830>] ? kthread_create_on_node+0x180/0x180
  [<ffffffff816f25a7>] ret_from_fork+0x47/0x90
  [<ffffffff810a6830>] ? kthread_create_on_node+0x180/0x180

The ownership rule is once the request is on the list, ownership transfers
to the list and the local thread can't touch it any more, just like for
the normal MAD case in send_mad().

Thus, instead of adding before send and then trying to delete after on
errors, move the entire thing under the spinlock so that the send and
update of the lists are atomic to the conurrent threads. Lightly reoganize
things so spinlock safe memory allocations are done in the final NL send
path and the rest of the setup work is done before and outside the lock.

Fixes: 3ebd2fd0 ("IB/sa: Put netlink request into the request list before sending")
Link: https://lore.kernel.org/r/1592964789-14533-1-git-send-email-divya.indi@oracle.comSigned-off-by: Divya Indi <divya.indi@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>