linux-kernel - Re: [PATCH] drm/etnaviv: fix memory leak when mapping prime imported buffers

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Date:   Wed, 20 May 2020 12:29:19 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Martin Fuzzey <martin.fuzzey@...wbird.group>
Cc:     stable@...r.kernel.org,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        etnaviv@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/etnaviv: fix memory leak when mapping prime
 imported buffers

Hi Martin,

Am Mittwoch, den 20.05.2020, 12:10 +0200 schrieb Martin Fuzzey:
> When using mmap() on a prime imported buffer allocated by a
> different driver (such as imx-drm) the later munmap() does
> not correctly decrement the refcount of the original enaviv_gem_object,
> leading to a leak.
> 
> Signed-off-by: Martin Fuzzey <martin.fuzzey@...wbird.group>
> Cc: stable@...r.kernel.org

What's the use-case where you did hit this issue? mmap'ing of imported
buffers through the etnaviv DRM device is currently not well defined
and I was pondering the idea of forbidding it completely by not
returning a mmap offset for those objects.

Regards,
Lucas

> ---
>  drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c | 20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c b/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> index f24dd21..28a01b8 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem_prime.c
> @@ -93,7 +93,25 @@ static void *etnaviv_gem_prime_vmap_impl(struct etnaviv_gem_object *etnaviv_obj)
>  static int etnaviv_gem_prime_mmap_obj(struct etnaviv_gem_object *etnaviv_obj,
>  		struct vm_area_struct *vma)
>  {
> -	return dma_buf_mmap(etnaviv_obj->base.dma_buf, vma, 0);
> +	int ret;
> +
> +	ret = dma_buf_mmap(etnaviv_obj->base.dma_buf, vma, 0);
> +
> +	/* drm_gem_mmap_obj() has already been called before this function
> +	 * and has incremented our refcount, expecting it to be decremented
> +	 * on unmap() via drm_gem_vm_close().
> +	 * However dma_buf_mmap() invokes drm_gem_cma_prime_mmap()
> +	 * that ends up updating the vma->vma_private_data to point to the
> +	 * dma_buf's gem object.
> +	 * Hence our gem object here will not have its refcount decremented
> +	 * when userspace does unmap().
> +	 * So decrement the refcount here to avoid a memory leak if the dma
> +	 * buf mapping was successful.
> +	 */
> +	if (!ret)
> +		drm_gem_object_put_unlocked(&etnaviv_obj->base);
> +
> +	return ret;
>  }
>  
>  static const struct etnaviv_gem_ops etnaviv_gem_prime_ops = {