[PATCH] drm: Check if primary mst is null

[Ville Syrjälä]

On Wed, Nov 07, 2018 at 06:11:30PM +0200, Stanislav Lisovskiy wrote:
> Unfortunately drm_dp_get_mst_branch_device which is called from both
> drm_dp_mst_handle_down_rep and drm_dp_mst_handle_up_rep seem to rely
> on that mgr->mst_primary is not NULL, which seem to be wrong as it can be
> cleared with simultaneous mode set, if probing fails or in other case.
> mgr->lock mutex doesn't protect against that as it might just get assigned to NULL
> right before, not simultaneously.
> There are currently bugs 107738, 108816 bugs which crash in
> drm_dp_get_mst_branch_device, caused by this issue.

Use the proper Bugzilla: tags for those. This also seems like cc:stable
material.

> 
> Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 5ff1d79b86c4..fb90ed4cdc3a 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -1273,6 +1273,12 @@ static struct drm_dp_mst_branch *drm_dp_get_mst_branch_device(struct drm_dp_mst_
>  	/* find the port by iterating down */
>  
>  	mutex_lock(&mgr->lock);
> +
> +	if (!mgr->mst_primary) {
> +		mstb = NULL;
> +		goto out;
> +	}
> +
>  	mstb = mgr->mst_primary;
>  

Or a bit simpler:

  mstb = mgr->mst_primary;
+ if (!mstb)
+ 	goto out;

ccing Lyude as well..

>  	for (i = 0; i < lct - 1; i++) {
> -- 
> 2.17.1
> 
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-- 
Ville Syrjälä
Intel