Re: [PATCH v3] IB/sa: Resolving use-after-free in ib_nl_send_msg

[Leon Romanovsky]

On Tue, Jun 16, 2020 at 10:56:53AM -0700, Divya Indi wrote:
> Hi Leon,
>
> Please find my comments inline -
>
> On 6/13/20 11:41 PM, Leon Romanovsky wrote:
> > On Tue, Jun 09, 2020 at 07:45:21AM -0700, Divya Indi wrote:
> >> Hi Leon,
> >>
> >> Thanks for taking the time to review.
> >>
> >> Please find my comments inline -
> >>
> >> On 6/9/20 12:00 AM, Leon Romanovsky wrote:
> >>> On Mon, Jun 08, 2020 at 07:46:16AM -0700, Divya Indi wrote:
> >>>> Commit 3ebd2fd0d011 ("IB/sa: Put netlink request into the request list before sending")'
> >>>> -
> >>>> 1. Adds the query to the request list before ib_nl_snd_msg.
> >>>> 2. Removes ib_nl_send_msg from within the spinlock which also makes it
> >>>> possible to allocate memory with GFP_KERNEL.
> >>>>
> >>>> 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 release the query causing a use after free situation
> >>>> while accessing the query in ib_nl_send_msg.
> >>>>
> >>>> Call Trace for the above race:
> >>>>
> >>>> [<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]
> >>>>
> >>>> To resolve the above issue -
> >>>> 1. Add the req to the request list only after the request has been sent out.
> >>>> 2. To handle the race where response comes in before adding request to
> >>>> the request list, send(rdma_nl_multicast) and add to list while holding the
> >>>> spinlock - request_lock.
> >>>> 3. Use GFP_NOWAIT for rdma_nl_multicast since it is called while holding
> >>>> a spinlock. In case of memory allocation failure, request will go out to SA.
> >>>>
> >>>> Signed-off-by: Divya Indi <divya.indi@xxxxxxxxxx>
> >>>> Fixes: 3ebd2fd0d011 ("IB/sa: Put netlink request into the request list
> >>>> before sending")
> >>> Author SOB should be after "Fixes" line.
> >> My bad. Noted.
> >>
> >>>> ---
> >>>>  drivers/infiniband/core/sa_query.c | 34 +++++++++++++++++-----------------
> >>>>  1 file changed, 17 insertions(+), 17 deletions(-)
> >>>>
> >>>> diff --git a/drivers/infiniband/core/sa_query.c b/drivers/infiniband/core/sa_query.c
> >>>> index 74e0058..042c99b 100644
> >>>> --- a/drivers/infiniband/core/sa_query.c
> >>>> +++ b/drivers/infiniband/core/sa_query.c
> >>>> @@ -836,6 +836,9 @@ static int ib_nl_send_msg(struct ib_sa_query *query, gfp_t gfp_mask)
> >>>>  	void *data;
> >>>>  	struct ib_sa_mad *mad;
> >>>>  	int len;
> >>>> +	unsigned long flags;
> >>>> +	unsigned long delay;
> >>>> +	int ret;
> >>>>
> >>>>  	mad = query->mad_buf->mad;
> >>>>  	len = ib_nl_get_path_rec_attrs_len(mad->sa_hdr.comp_mask);
> >>>> @@ -860,35 +863,32 @@ static int ib_nl_send_msg(struct ib_sa_query *query, gfp_t gfp_mask)
> >>>>  	/* Repair the nlmsg header length */
> >>>>  	nlmsg_end(skb, nlh);
> >>>>
> >>>> -	return rdma_nl_multicast(&init_net, skb, RDMA_NL_GROUP_LS, gfp_mask);
> >>>> +	spin_lock_irqsave(&ib_nl_request_lock, flags);
> >>>> +	ret =  rdma_nl_multicast(&init_net, skb, RDMA_NL_GROUP_LS, GFP_NOWAIT);
> >>> It is hard to be convinced that this is correct solution. The mix of
> >>> gfp_flags and GFP_NOWAIT at the same time and usage of
> >>> ib_nl_request_lock to protect lists and suddenly rdma_nl_multicast() too
> >>> makes this code unreadable/non-maintainable.
> >> Prior to 3ebd2fd0d011 ("IB/sa: Put netlink request into the request list
> >> before sending"), we had ib_nl_send_msg under the spinlock ib_nl_request_lock.
> >>
> >> ie we had -
> >>
> >> 1. Get spinlock - ib_nl_request_lock
> >> 2. ib_nl_send_msg
> >> 	2.a) rdma_nl_multicast
> >> 3. Add request to the req list
> >> 4. Arm the timer if needed.
> >> 5. Release spinlock
> >>
> >> However, ib_nl_send_msg involved a memory allocation using GFP_KERNEL.
> >> hence, was moved out of the spinlock. In addition, req was now being
> >> added prior to ib_nl_send_msg [To handle the race where response can
> >> come in before we get a chance to add the request back to the list].
> >>
> >> This introduced another race resulting in use-after-free.[Described in the commit.]
> >>
> >> To resolve this, sending out the request and adding it to list need to
> >> happen while holding the request_lock.
> >> To ensure minimum allocations while holding the lock, instead of having
> >> the entire ib_nl_send_msg under the lock, we only have rdma_nl_multicast
> >> under this spinlock.
> >>
> >> However, do you think it would be a good idea to split ib_nl_send_msg
> >> into 2 functions -
> >> 1. Prepare the req/query [Outside the spinlock]
> >> 2. Sending the req - rdma_nl_multicast [while holding spinlock]
> >>
> >> Would this be more intuitive?
> > While it is always good idea to minimize the locked period. It still
> > doesn't answer concern about mixing gfp_flags and direct GFP_NOWAIT.
> > For example if user provides GFP_ATOMIC, the GFP_NOWAIT allocation will
> > cause a trouble because latter is more lax than first one.
>
> Makes sense, and we do have callers passing GFP_ATOMIC with gfp_mask.
>
> However, in this case when we fail to send the request to ibacm,
> we then fallback to sending it to the SA with gfp_mask. So, the
> request will eventually go out with GFP_ATOMIC to SA. From the
> caller perspective the request will not fail due to memory pressure.
>
> -------
> send_mad(...gfp_mask)
> 	- send to ibacm with GFP_NOWAIT
> 	- If fails, send to SA with gfp_mask
> -------
>
> So, using GFP_NOWAIT may not cause trouble here.
>
> The other option might be to use GFP_NOWAIT conditionally ie
> (only use GFP_NOWAIT when GFP_ATOMIC is not specified in gfp_mask else
> use GFP_ATOMIC). Eventual goal being to not have a blocking memory allocation.
>
> Your thoughts?

My thoughts that everything here hints me that state machine and
locking are implemented wrongly. In ideal world, the expectation
is that REQ message will have a state in it (PREPARED, SENT, ACK
e.t.c.) and list manipulations are done accordingly with proper
locks, while rdma_nl_multicast() is done outside of the locks.

I don't know if it is possible to fix.

>
> Really appreciate your feedback. Thanks!
>
>
> Regards,
> Divya
>
> >
> > Thanks
> >
> >>>> +	if (!ret) {
> >>> Please use kernel coding style.
> >>>
> >>> if (ret) {
> >>>   spin_unlock_irqrestore(&ib_nl_request_lock, flags);
> >>>   return ret;
> >>>   }
> >>>
> >>>  ....
> >> Noted. Will make this change.
> >>
> >>>> +		/* Put the request on the list.*/
> >>>> +		delay = msecs_to_jiffies(sa_local_svc_timeout_ms);
> >>>> +		query->timeout = delay + jiffies;
> >>>> +		list_add_tail(&query->list, &ib_nl_request_list);
> >>>> +		/* Start the timeout if this is the only request */
> >>>> +		if (ib_nl_request_list.next == &query->list)
> >>>> +			queue_delayed_work(ib_nl_wq, &ib_nl_timed_work, delay);
> >>>> +	}
> >>>> +	spin_unlock_irqrestore(&ib_nl_request_lock, flags);
> >>>> +
> >>>> +	return ret;
> >>>>  }
> >>>>
> >>>>  static int ib_nl_make_request(struct ib_sa_query *query, gfp_t gfp_mask)
> >>>>  {
> >>>> -	unsigned long flags;
> >>>> -	unsigned long delay;
> >>>>  	int ret;
> >>>>
> >>>>  	INIT_LIST_HEAD(&query->list);
> >>>>  	query->seq = (u32)atomic_inc_return(&ib_nl_sa_request_seq);
> >>>>
> >>>> -	/* Put the request on the list first.*/
> >>>> -	spin_lock_irqsave(&ib_nl_request_lock, flags);
> >>>> -	delay = msecs_to_jiffies(sa_local_svc_timeout_ms);
> >>>> -	query->timeout = delay + jiffies;
> >>>> -	list_add_tail(&query->list, &ib_nl_request_list);
> >>>> -	/* Start the timeout if this is the only request */
> >>>> -	if (ib_nl_request_list.next == &query->list)
> >>>> -		queue_delayed_work(ib_nl_wq, &ib_nl_timed_work, delay);
> >>>> -	spin_unlock_irqrestore(&ib_nl_request_lock, flags);
> >>>> -
> >>>>  	ret = ib_nl_send_msg(query, gfp_mask);
> >>>>  	if (ret) {
> >>>>  		ret = -EIO;
> >>>> -		/* Remove the request */
> >>>> -		spin_lock_irqsave(&ib_nl_request_lock, flags);
> >>>> -		list_del(&query->list);
> >>>> -		spin_unlock_irqrestore(&ib_nl_request_lock, flags);
> >>>>  	}
> >>> Brackets should be removed too.
> >> Noted.
> >>>>  	return ret;
> >>>> --
> >>>> 1.8.3.1
> >>>>