Re: [PATCH 1/2] IB/sa: Resolving use-after-free in ib_nl_send_msg.

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

Hi,

Thanks for taking the time to review. Please find my comments inline -

On 5/7/20 1:16 PM, Wan, Kaike wrote:
>
>> -----Original Message-----
>> From: Mark Bloch <markb@mellanox.com>
>> Sent: Thursday, May 07, 2020 3:36 PM
>> To: Divya Indi <divya.indi@oracle.com>; linux-kernel@vger.kernel.org; linux-
>> rdma@vger.kernel.org; Jason Gunthorpe <jgg@ziepe.ca>; Wan, Kaike
>> <kaike.wan@intel.com>
>> Cc: Gerd Rausch <gerd.rausch@oracle.com>; Håkon Bugge
>> <haakon.bugge@oracle.com>; Srinivas Eeda <srinivas.eeda@oracle.com>;
>> Rama Nichanamatlu <rama.nichanamatlu@oracle.com>; Doug Ledford
>> <dledford@redhat.com>
>> 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. 

We could use a lock or use atomic operations. However, the reason for not doing so was that
we have 1 writer and multiple readers of the IB_SA_NL_QUERY_SENT flag and the readers 
wouldnt mind reading a stale value. 

Would it still be better to have a lock for this flag? 

Thanks,
Divya

>
> Kaike
>