linux-kernel - Re: [PATCH] dma-buf: fix debugfs versus rcu and fence dumping

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Date:   Thu, 6 Dec 2018 16:08:12 +0000
From:   "Koenig, Christian" <Christian.Koenig@....com>
To:     Jerome Glisse <jglisse@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        Stéphane Marchesin <marcheu@...omium.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] dma-buf: fix debugfs versus rcu and fence dumping

Am 06.12.18 um 16:21 schrieb Jerome Glisse:
> On Thu, Dec 06, 2018 at 08:09:28AM +0000, Koenig, Christian wrote:
>> Am 06.12.18 um 02:41 schrieb jglisse@...hat.com:
>>> From: Jérôme Glisse <jglisse@...hat.com>
>>>
>>> The debugfs take reference on fence without dropping them. Also the
>>> rcu section are not well balance. Fix all that ...
>>>
>>> Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
>>> Cc: Christian König <christian.koenig@....com>
>>> Cc: Daniel Vetter <daniel.vetter@...ll.ch>
>>> Cc: Sumit Semwal <sumit.semwal@...aro.org>
>>> Cc: linux-media@...r.kernel.org
>>> Cc: dri-devel@...ts.freedesktop.org
>>> Cc: linaro-mm-sig@...ts.linaro.org
>>> Cc: Stéphane Marchesin <marcheu@...omium.org>
>>> Cc: stable@...r.kernel.org
>> Well NAK, you are now taking the RCU lock twice and dropping the RCU and
>> still accessing fobj has a huge potential for accessing freed up memory.
>>
>> The only correct thing I can see here is to grab a reference to the
>> fence before printing any info on it,
>> Christian.
> Hu ? That is exactly what i am doing, take reference under rcu,
> rcu_unlock print the fence info, drop the fence reference, rcu
> lock rinse and repeat ...
>
> Note that the fobj in _existing_ code is access outside the rcu
> end that there is an rcu imbalance in that code ie a lonlely
> rcu_unlock after the for loop.
>
> So that the existing code is broken.

No, the existing code is perfectly fine.

Please note the break in the loop before the rcu_unlock();
>    			if (!read_seqcount_retry(&robj->seq, seq))
>    				break; <- HERE!
>    			rcu_read_unlock();
>    		}

So your patch breaks that and take the RCU read lock twice.

Regards,
Christian.

>
>>> ---
>>>    drivers/dma-buf/dma-buf.c | 11 +++++++++--
>>>    1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>>> index 13884474d158..f6f4de42ac49 100644
>>> --- a/drivers/dma-buf/dma-buf.c
>>> +++ b/drivers/dma-buf/dma-buf.c
>>> @@ -1051,24 +1051,31 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
>>>    			fobj = rcu_dereference(robj->fence);
>>>    			shared_count = fobj ? fobj->shared_count : 0;
>>>    			fence = rcu_dereference(robj->fence_excl);
>>> +			fence = dma_fence_get_rcu(fence);
>>>    			if (!read_seqcount_retry(&robj->seq, seq))
>>>    				break;
>>>    			rcu_read_unlock();
>>>    		}
>>> -
>>> -		if (fence)
>>> +		if (fence) {
>>>    			seq_printf(s, "\tExclusive fence: %s %s %ssignalled\n",
>>>    				   fence->ops->get_driver_name(fence),
>>>    				   fence->ops->get_timeline_name(fence),
>>>    				   dma_fence_is_signaled(fence) ? "" : "un");
>>> +			dma_fence_put(fence);
>>> +		}
>>> +
>>> +		rcu_read_lock();
>>>    		for (i = 0; i < shared_count; i++) {
>>>    			fence = rcu_dereference(fobj->shared[i]);
>>>    			if (!dma_fence_get_rcu(fence))
>>>    				continue;
>>> +			rcu_read_unlock();
>>>    			seq_printf(s, "\tShared fence: %s %s %ssignalled\n",
>>>    				   fence->ops->get_driver_name(fence),
>>>    				   fence->ops->get_timeline_name(fence),
>>>    				   dma_fence_is_signaled(fence) ? "" : "un");
>>> +			dma_fence_put(fence);
>>> +			rcu_read_lock();
>>>    		}
>>>    		rcu_read_unlock();
>>>