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

[Leon Romanovsky <leon@kernel.org>]

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@oracle.com>
> >> 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.

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
> >>