[PATCH] drm/dp_mst: Avoid NULL pointer dereference

[Lyude Paul]

Back from the holidays!

Reviewed-by: Lyude Paul <lyude at redhat.com>

Do you need me to push this to drm-misc?

On Thu, 2019-12-26 at 10:31 +0800, Wayne Lin wrote:
> [Why]
> Found kernel NULL pointer dereference under the below situation:
> 
> 	src — HDMI_Monitor   src — HDMI_Monitor
> e.g.:	    \            =>
> 	     MSTB — MSTB     (unplug) MSTB — MSTB
> 
> When display 1 HDMI and 2 DP daisy chain monitors, unplugging the dp
> cable connected to source causes kernel NULL pointer dereference at
> drm_dp_mst_atomic_check_bw_limit(). When calculating pbn_limit, if
> branch is null, accessing "&branch->ports" causes the problem.
> 
> [How]
> Judge branch is null or not at the beginning. If it is null, return 0.
> 
> Signed-off-by: Wayne Lin <Wayne.Lin at amd.com>
> Cc: stable at vger.kernel.org
> ---
>  drivers/gpu/drm/drm_dp_mst_topology.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> b/drivers/gpu/drm/drm_dp_mst_topology.c
> index 7d2d31eaf003..a6473e3ab448 100644
> --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> @@ -4707,6 +4707,9 @@ int drm_dp_mst_atomic_check_bw_limit(struct
> drm_dp_mst_branch *branch,
>  	struct drm_dp_vcpi_allocation *vcpi;
>  	int pbn_limit = 0, pbn_used = 0;
>  
> +	if (!branch)
> +		return 0;
> +
>  	list_for_each_entry(port, &branch->ports, next) {
>  		if (port->mstb)
>  			if (drm_dp_mst_atomic_check_bw_limit(port->mstb,
> mst_state))
-- 
Cheers,
	Lyude Paul