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

[Jerome Glisse <jglisse@redhat.com>]

On Thu, Dec 06, 2018 at 08:09:28AM +0000, Koenig, Christian wrote:
> Am 06.12.18 um 02:41 schrieb jglisse@redhat.com:
> > From: Jérôme Glisse <jglisse@redhat.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@redhat.com>
> > Cc: Christian König <christian.koenig@amd.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > Cc: linux-media@vger.kernel.org
> > Cc: dri-devel@lists.freedesktop.org
> > Cc: linaro-mm-sig@lists.linaro.org
> > Cc: Stéphane Marchesin <marcheu@chromium.org>
> > Cc: stable@vger.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.

> 
> > ---
> >   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();
> >   
>