Request for feedback : Possible use-after-free in routing SA query via netlink

[Divya Indi <divya.indi@oracle.com>]

Hi All,

I wanted some feedback on a crash caused due to use-after-free in the 
ibacm code path [while routing SA query via netlink].

Commit 3ebd2fd IB/sa: Put netlink request into the request list before sending

Above commit moved adding the query to the request list before ib_nl_snd_msg
and moved ib_nl_snd_msg out of the spinlock (request_lock).

However, if there is a delay in sending out the request (For
eg: Delay due to low memory situation) the timer to handle request timeout
might kick in before the request is sent out to ibacm via netlink.
ib_nl_request_timeout may result in release of the query (by call to send_handler) 
while ib_nl_snd_msg is still accessing query.


We get the following stacktrace for the crash -

[<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
....
RIP  [<ffffffffa03296cd>] send_mad+0x33d/0x5d0 [ib_sa] 

On analysis of the vmcore, we see crash happens at -

ib_sa_path_rec_get
  send_mad
     ib_nl_make_request()
        ib_nl_send_msg
                1  static void ib_nl_set_path_rec_attrs(struct sk_buff *skb,
                2        struct ib_sa_query *query)
                3  {
                4   struct ib_sa_path_rec *sa_rec = query->mad_buf->context[1];
                5   struct ib_sa_mad *mad = query->mad_buf->mad;
                6   ib_sa_comp_mask comp_mask = mad->sa_hdr.comp_mask;

Page fault occurs at line 5 while trying to access query->mad_buf->mad;

If we look at the query, it does not appear to be a valid ib_sa_query. Instead
looks like a pid struct for a process -> Use-after-free situation.

We could simulate the crash by explicitly introducing a delay in ib_nl_snd_msg with
a sleep. The timer kicks in before ib_nl_send_msg has even sent out the request 
and releases the query. We could reproduce the crash with a similar stack trace.

To summarize - We have a use-after-free possibility here when the timer(ib_nl_request_timeout)
kicks in before ib_nl_snd_msg has completed sending the query out to ibacm via netlink. The 
timeout handler ie ib_nl_request_timeout may result in releasing the query while ib_nl_snd_msg 
is still accessing query.

Appreciate your thoughts on the above issue.


Thanks,
Divya