Re: [PATCH v2] drm/display/dp_mst: Add protection against 0 vcpi

[Lyude Paul <lyude@redhat.com>]

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

On Wed, 2025-11-19 at 15:16 +0530, Suraj Kandpal wrote:
> When releasing a timeslot there is a slight chance we may end up
> with the wrong payload mask due to overflow if the delayed_destroy_work
> ends up coming into play after a DP 2.1 monitor gets disconnected
> which causes vcpi to become 0 then we try to make the payload =
> ~BIT(vcpi - 1) which is a negative shift. VCPI id should never
> really be 0 hence skip changing the payload mask if VCPI is 0.
> 
> Otherwise it leads to
> <7> [515.287237] xe 0000:03:00.0: [drm:drm_dp_mst_get_port_malloc
> [drm_display_helper]] port ffff888126ce9000 (3)
> <4> [515.287267] -----------[ cut here ]-----------
> <3> [515.287268] UBSAN: shift-out-of-bounds in
> ../drivers/gpu/drm/display/drm_dp_mst_topology.c:4575:36
> <3> [515.287271] shift exponent -1 is negative
> <4> [515.287275] CPU: 7 UID: 0 PID: 3108 Comm: kworker/u64:33 Tainted: G
> S U 6.17.0-rc6-lgci-xe-xe-3795-3e79699fa1b216e92+ #1 PREEMPT(voluntary)
> <4> [515.287279] Tainted: [S]=CPU_OUT_OF_SPEC, [U]=USER
> <4> [515.287279] Hardware name: ASUS System Product Name/PRIME Z790-P
> WIFI, BIOS 1645 03/15/2024
> <4> [515.287281] Workqueue: drm_dp_mst_wq drm_dp_delayed_destroy_work
> [drm_display_helper]
> <4> [515.287303] Call Trace:
> <4> [515.287304] <TASK>
> <4> [515.287306] dump_stack_lvl+0xc1/0xf0
> <4> [515.287313] dump_stack+0x10/0x20
> <4> [515.287316] __ubsan_handle_shift_out_of_bounds+0x133/0x2e0
> <4> [515.287324] ? drm_atomic_get_private_obj_state+0x186/0x1d0
> <4> [515.287333] drm_dp_atomic_release_time_slots.cold+0x17/0x3d
> [drm_display_helper]
> <4> [515.287355] mst_connector_atomic_check+0x159/0x180 [xe]
> <4> [515.287546] drm_atomic_helper_check_modeset+0x4d9/0xfa0
> <4> [515.287550] ? __ww_mutex_lock.constprop.0+0x6f/0x1a60
> <4> [515.287562] intel_atomic_check+0x119/0x2b80 [xe]
> <4> [515.287740] ? find_held_lock+0x31/0x90
> <4> [515.287747] ? lock_release+0xce/0x2a0
> <4> [515.287754] drm_atomic_check_only+0x6a2/0xb40
> <4> [515.287758] ? drm_atomic_add_affected_connectors+0x12b/0x140
> <4> [515.287765] drm_atomic_commit+0x6e/0xf0
> <4> [515.287766] ? _pfx__drm_printfn_info+0x10/0x10
> <4> [515.287774] drm_client_modeset_commit_atomic+0x25c/0x2b0
> <4> [515.287794] drm_client_modeset_commit_locked+0x60/0x1b0
> <4> [515.287795] ? mutex_lock_nested+0x1b/0x30
> <4> [515.287801] drm_client_modeset_commit+0x26/0x50
> <4> [515.287804] __drm_fb_helper_restore_fbdev_mode_unlocked+0xdc/0x110
> <4> [515.287810] drm_fb_helper_hotplug_event+0x120/0x140
> <4> [515.287814] drm_fbdev_client_hotplug+0x28/0xd0
> <4> [515.287819] drm_client_hotplug+0x6c/0xf0
> <4> [515.287824] drm_client_dev_hotplug+0x9e/0xd0
> <4> [515.287829] drm_kms_helper_hotplug_event+0x1a/0x30
> <4> [515.287834] drm_dp_delayed_destroy_work+0x3df/0x410
> [drm_display_helper]
> <4> [515.287861] process_one_work+0x22b/0x6f0
> <4> [515.287874] worker_thread+0x1e8/0x3d0
> <4> [515.287879] ? __pfx_worker_thread+0x10/0x10
> <4> [515.287882] kthread+0x11c/0x250
> <4> [515.287886] ? __pfx_kthread+0x10/0x10
> <4> [515.287890] ret_from_fork+0x2d7/0x310
> <4> [515.287894] ? __pfx_kthread+0x10/0x10
> <4> [515.287897] ret_from_fork_asm+0x1a/0x30
> 
> Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6303
> Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
> ---
> v1 -> v2:
> -Add the call trace and closes tag [Jani]
> -Change payload mask only is vcpi > 0 [Imre]
> 
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index 64e5c176d5cc..be749dcad3b5 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -4572,7 +4572,8 @@ int drm_dp_atomic_release_time_slots(struct drm_atomic_state *state,
>  	if (!payload->delete) {
>  		payload->pbn = 0;
>  		payload->delete = true;
> -		topology_state->payload_mask &= ~BIT(payload->vcpi - 1);
> +		if (payload->vcpi > 0)
> +			topology_state->payload_mask &= ~BIT(payload->vcpi - 1);
>  	}
>  
>  	return 0;

-- 
Cheers,
 Lyude Paul (she/her)
 Senior Software Engineer at Red Hat

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