Re: [PATCH v7 2/2] drm/gem: Don't map imported GEMs

[Thomas Hellström (Intel)]

Hi, Dmitry,

On 6/30/22 22:04, Dmitry Osipenko wrote:
> Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
> handle imported dma-bufs properly, which results in mapping of something
> else than the imported dma-buf. On NVIDIA Tegra we get a hard lockup when
> userspace writes to the memory mapping of a dma-buf that was imported into
> Tegra's DRM GEM.
>
> Majority of DRM drivers prohibit mapping of the imported GEM objects.
> Mapping of imported GEMs require special care from userspace since it
> should sync dma-buf because mapping coherency of the exporter device may
> not match the DRM device. Let's prohibit the mapping for all DRM drivers
> for consistency.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Suggested-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>

This might break drivers whose obj->funcs->mmap() callback already 
handles this case, and has userspace that does the right thing.

I think the disabling must be checked on a per-driver basis to avoid 
that; in particular drivers that already call dma_buf_mmap() should be 
able to continue doing this.

Also the Fixes: review comment remains,

thanks,

Thomas



> ---
>   drivers/gpu/drm/drm_gem.c              | 4 ++++
>   drivers/gpu/drm/drm_gem_shmem_helper.c | 9 ---------
>   2 files changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 86d670c71286..fc9ec42fa0ab 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -1034,6 +1034,10 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
>   {
>   	int ret;
>   
> +	/* Don't allow imported objects to be mapped */
> +	if (obj->import_attach)
> +		return -EINVAL;
> +
>   	/* Check for valid size. */
>   	if (obj_size < vma->vm_end - vma->vm_start)
>   		return -EINVAL;
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 8ad0e02991ca..6190f5018986 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -609,17 +609,8 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_vm_ops);
>    */
>   int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct *vma)
>   {
> -	struct drm_gem_object *obj = &shmem->base;
>   	int ret;
>   
> -	if (obj->import_attach) {
> -		/* Drop the reference drm_gem_mmap_obj() acquired.*/
> -		drm_gem_object_put(obj);
> -		vma->vm_private_data = NULL;
> -
> -		return dma_buf_mmap(obj->dma_buf, vma, 0);
> -	}
> -
>   	ret = drm_gem_shmem_get_pages(shmem);
>   	if (ret) {
>   		drm_gem_vm_close(vma);