Re: [PATCH v15 01/23] drm/shmem-helper: Fix UAF in error path when freeing SGT of imported GEM

[Boris Brezillon]

On Sun, 27 Aug 2023 20:54:27 +0300
Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:

> Freeing drm-shmem GEM right after creating it using
> drm_gem_shmem_prime_import_sg_table() frees SGT of the imported dma-buf
> and then dma-buf frees this SGT second time.
> 
> The v3d_prime_import_sg_table() is example of a error code path where
> dma-buf's SGT is freed by drm-shmem and then it's freed second time by
> dma_buf_unmap_attachment() in drm_gem_prime_import_dev().
> 
> Add drm-shmem GEM flag telling that this is imported SGT shall not be
> treated as own SGT, fixing the use-after-free bug.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ++-
>  include/drm/drm_gem_shmem_helper.h     | 7 +++++++
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index a783d2245599..78d9cf2355a5 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -141,7 +141,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>  
>  	if (obj->import_attach) {
>  		drm_prime_gem_destroy(obj, shmem->sgt);
> -	} else {
> +	} else if (!shmem->imported_sgt) {
>  		dma_resv_lock(shmem->base.resv, NULL);
>  
>  		drm_WARN_ON(obj->dev, shmem->vmap_use_count);
> @@ -758,6 +758,7 @@ drm_gem_shmem_prime_import_sg_table(struct drm_device *dev,
>  		return ERR_CAST(shmem);
>  
>  	shmem->sgt = sgt;
> +	shmem->imported_sgt = true;


I feel like adding more fields that can be used to do the is_imported()
check is going to be even more confusing. Can we instead have

	/* drm_gem_shmem_prime_import_sg_table() can be called from a
	 * driver specific ->import_sg_table() implementations that
	 * have extra failable initialization steps. Assign
	 * drm_gem_object::import_attach here (even though it's
	 * assigned in drm_gem_prime_import_dev()), so we don't end up
	 * with driver error paths calling drm_gem_shmem_free() with an
	 * imported sg_table assigned to drm_gem_shmem_object::sgt and
	 * drm_gem_object::import_attach left uninitialized.
	 */
	shmem->base.import_attach = attach;

here?

>  
>  	drm_dbg_prime(dev, "size = %zu\n", size);
>  
> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_shmem_helper.h
> index bf0c31aa8fbe..ec70a98a8fe1 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -73,6 +73,13 @@ struct drm_gem_shmem_object {
>  	 */
>  	unsigned int vmap_use_count;
>  
> +	/**
> +	 * @imported_sgt:
> +	 *
> +	 * True if SG table belongs to imported dma-buf.
> +	 */
> +	bool imported_sgt : 1;
> +
>  	/**
>  	 * @pages_mark_dirty_on_put:
>  	 *