> -----Original Message-----
> From: Mark Bloch <markb@xxxxxxxxxxxx>
> Sent: Thursday, May 07, 2020 3:36 PM
> To: Divya Indi <divya.indi@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; linux-
> rdma@xxxxxxxxxxxxxxx; Jason Gunthorpe <jgg@xxxxxxxx>; Wan, Kaike
> <kaike.wan@xxxxxxxxx>
> Cc: Gerd Rausch <gerd.rausch@xxxxxxxxxx>; Håkon Bugge
> <haakon.bugge@xxxxxxxxxx>; Srinivas Eeda <srinivas.eeda@xxxxxxxxxx>;
> Rama Nichanamatlu <rama.nichanamatlu@xxxxxxxxxx>; Doug Ledford
> <dledford@xxxxxxxxxx>
> Subject: Re: [PATCH 1/2] IB/sa: Resolving use-after-free in ib_nl_send_msg.
>
>
> > @@ -1123,6 +1156,18 @@ int ib_nl_handle_resolve_resp(struct sk_buff
> > *skb,
> >
> > send_buf = query->mad_buf;
> >
> > + /*
> > + * Make sure the IB_SA_NL_QUERY_SENT flag is set before
> > + * processing this query. If flag is not set, query can be accessed in
> > + * another context while setting the flag and processing the query
> will
> > + * eventually release it causing a possible use-after-free.
> > + */
> > + if (unlikely(!ib_sa_nl_query_sent(query))) {
>
> Can't there be a race here where you check the flag (it isn't set) and before
> you call wait_event() the flag is set and wake_up() is called which means you
> will wait here forever?
Should wait_event() catch that? That is, if the flag is not set, wait_event() will sleep until the flag is set.
or worse, a timeout will happen the query will be
> freed and them some other query will call wake_up() and we have again a
> use-after-free.
The request has been deleted from the request list by this time and therefore the timeout should have no impact here.
>
> > + spin_unlock_irqrestore(&ib_nl_request_lock, flags);
> > + wait_event(wait_queue, ib_sa_nl_query_sent(query));
>
> What if there are two queries sent to userspace, shouldn't you check and
> make sure you got woken up by the right one setting the flag?
The wait_event() is conditioned on the specific query (ib_sa_nl_query_sent(query)), not on the wait_queue itself.
>
> Other than that, the entire solution makes it very complicated to reason with
> (flags set/checked without locking etc) maybe we should just revert and fix it
> the other way?
The flag could certainly be set under the lock, which may reduce complications.
Kaike
