linux-kernel - Re: [PATCH] drm/prime: fix a potential double put (release) bug

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Message-ID: <14aa6dfe-faba-8632-01a4-8119f199005c@amd.com>
Date:   Wed, 18 Aug 2021 15:25:59 +0200
From:   Christian König <christian.koenig@....com>
To:     Wentao_Liang <Wentao_Liang_g@....com>,
        maarten.lankhorst@...ux.intel.com
Cc:     mripard@...nel.org, tzimmermann@...e.de, airlied@...ux.ie,
        daniel@...ll.ch, sumit.semwal@...aro.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH] drm/prime: fix a potential double put (release) bug

Am 18.08.21 um 15:02 schrieb Wentao_Liang:
> In line 317 (#1), drm_gem_prime_import() is called, it will call
> drm_gem_prime_import_dev(). At the end of the function
> drm_gem_prime_import_dev() (line 956, #2), "dma_buf_put(dma_buf);" puts
> dma_buf->file and may cause it to be released. However, after
> drm_gem_prime_import() returning, the dma_buf may be put again by the
> same put function in lines 342, 351 and 358 (#3, #4, #5). Putting the
> dma_buf improperly more than once can lead to an incorrect dma_buf-
>> file put.
> We believe that the put of the dma_buf in the function
> drm_gem_prime_import() is unnecessary (#2). We can fix the above bug by
> removing the redundant "dma_buf_put(dma_buf);" in line 956.

Guys I'm getting tired of NAKing those incorrect reference count analysis.

The dma_buf_put() in the error handling of drm_gem_prime_import_dev() 
function is balanced with the get_dma_buf() in the same function 
directly above.

This is for the creating a GEM object for a DMA-buf imported from other 
device use case and certainly correct.

The various dma_buf_put() in drm_gem_prime_fd_to_handle() is balanced 
with the dma_buf_get(prime_fd) at the beginning of the function.

This is for extracting the DMA-buf from the file descriptor and keeping 
a reference to it while we are busy importing it (e.g. to prevent a race 
when somebody changes the fd at the same time).

As far as I can see this is correct as well.

Regards,
Christian.

>
>   314     if (dev->driver->gem_prime_import)
>   315         obj = dev->driver->gem_prime_import(dev, dma_buf);
>   316     else
>   317         obj = drm_gem_prime_import(dev, dma_buf);
>   				//#1 call to drm_gem_prime_import
> 				//   ->drm_gem_prime_import_dev
> 				//   ->dma_buf_put
>   ...
>
>   336     ret = drm_prime_add_buf_handle(&file_priv->prime,
>   337             dma_buf, *handle);
>
>   ...
>
>   342     dma_buf_put(dma_buf);  //#3 put again
>   343
>   344     return 0;
>   345
>   346 fail:
>
>   351     dma_buf_put(dma_buf); //#4 put again
>   352     return ret;
>
>   356 out_put:
>   357     mutex_unlock(&file_priv->prime.lock);
>   358     dma_buf_put(dma_buf);  //#5 put again
>   359     return ret;
>   360 }
>
>   905 struct drm_gem_object *drm_gem_prime_import_dev
>   							(struct drm_device *dev,
>   906                         struct dma_buf *dma_buf,
>   907                         struct device *attach_dev)
>   908 {
>
>   ...
>
>   952 fail_unmap:
>   953     dma_buf_unmap_attachment(attach, sgt, DMA_BIDIRECTIONAL);
>   954 fail_detach:
>   955     dma_buf_detach(dma_buf, attach);
>   956     dma_buf_put(dma_buf);  //#2 the first put of dma_buf
> 								//	 (unnecessary)
>   957
>   958     return ERR_PTR(ret);
>   959 }
>
> Signed-off-by: Wentao_Liang <Wentao_Liang_g@....com>
> ---
>   drivers/gpu/drm/drm_prime.c | 1 -
>   1 file changed, 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 2a54f86856af..cef03ad0d5cd 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -953,7 +953,6 @@ struct drm_gem_object *drm_gem_prime_import_dev(struct drm_device *dev,
>   	dma_buf_unmap_attachment(attach, sgt, DMA_BIDIRECTIONAL);
>   fail_detach:
>   	dma_buf_detach(dma_buf, attach);
> -	dma_buf_put(dma_buf);
>   
>   	return ERR_PTR(ret);
>   }