linux-kernel - Re: [PATCH 1/4] staging: android: ion: Support cpu access during dma_buf

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Date:   Fri, 18 Jan 2019 13:34:16 -0600
From:   "Andrew F. Davis" <afd@...com>
To:     Liam Mark <lmark@...eaurora.org>, <labbott@...hat.com>,
        <sumit.semwal@...aro.org>
CC:     <arve@...roid.com>, <tkjos@...roid.com>, <maco@...roid.com>,
        <joel@...lfernandes.org>, <christian@...uner.io>,
        <devel@...verdev.osuosl.org>, <dri-devel@...ts.freedesktop.org>,
        <linaro-mm-sig@...ts.linaro.org>, <linux-kernel@...r.kernel.org>,
        <john.stultz@...aro.org>
Subject: Re: [PATCH 1/4] staging: android: ion: Support cpu access during
 dma_buf_detach

On 1/18/19 12:37 PM, Liam Mark wrote:
> Often userspace doesn't know when the kernel will be calling dma_buf_detach
> on the buffer.
> If userpace starts its CPU access at the same time as the sg list is being
> freed it could end up accessing the sg list after it has been freed.
> 
> Thread A				Thread B
> - DMA_BUF_IOCTL_SYNC IOCT
>  - ion_dma_buf_begin_cpu_access
>   - list_for_each_entry
> 					- ion_dma_buf_detatch
> 					 - free_duped_table
>    - dma_sync_sg_for_cpu
> 

The window for this seems really small, but it does seem technically
possible, good find. for what it's worth:

Acked-by: Andrew F. Davis <afd@...com>

> Fix this by getting the ion_buffer lock before freeing the sg table memory.
> 
> Fixes: 2a55e7b5e544 ("staging: android: ion: Call dma_map_sg for syncing and mapping")
> Signed-off-by: Liam Mark <lmark@...eaurora.org>
> ---
>  drivers/staging/android/ion/ion.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
> index a0802de8c3a1..6f5afab7c1a1 100644
> --- a/drivers/staging/android/ion/ion.c
> +++ b/drivers/staging/android/ion/ion.c
> @@ -248,10 +248,10 @@ static void ion_dma_buf_detatch(struct dma_buf *dmabuf,
>  	struct ion_dma_buf_attachment *a = attachment->priv;
>  	struct ion_buffer *buffer = dmabuf->priv;
>  
> -	free_duped_table(a->table);
>  	mutex_lock(&buffer->lock);
>  	list_del(&a->list);
>  	mutex_unlock(&buffer->lock);
> +	free_duped_table(a->table);
>  
>  	kfree(a);
>  }
>