Re: [PATCH v2 1/3] drm: bridge/panel: Cleanup connector on bridge detach

From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Paul Cercueil <paul@crapouillou.net>,
	Jernej Skrabec <jernej.skrabec@siol.net>,
	Neil Armstrong <narmstrong@baylibre.com>,
	David Airlie <airlied@linux.ie>, Jonas Karlman <jonas@kwiboo.se>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Andrzej Hajda <a.hajda@samsung.com>,
	od@zcrc.me, stable <stable@vger.kernel.org>,
	Sam Ravnborg <sam@ravnborg.org>
Subject: Re: [PATCH v2 1/3] drm: bridge/panel: Cleanup connector on bridge detach
Date: Wed, 24 Mar 2021 04:15:37 +0200	[thread overview]
Message-ID: <YFqgyTNt42vBe+w+@pendragon.ideasonboard.com> (raw)
In-Reply-To: <CAKMK7uFHYPvJm46f-LXBO=nERGBBO3i_=YXZyAUi0ZXJFLmXVw@mail.gmail.com>

On Wed, Jan 20, 2021 at 06:38:03PM +0100, Daniel Vetter wrote:
> On Wed, Jan 20, 2021 at 6:12 PM Paul Cercueil wrote:
> > Le mer. 20 janv. 2021 à 17:03, Daniel Vetter a écrit :
> > > On Wed, Jan 20, 2021 at 1:35 PM Paul Cercueil wrote:
> > >>
> > >>  If we don't call drm_connector_cleanup() manually in
> > >>  panel_bridge_detach(), the connector will be cleaned up with the other
> > >>  DRM objects in the call to drm_mode_config_cleanup(). However, since our
> > >>  drm_connector is devm-allocated, by the time drm_mode_config_cleanup()
> > >>  will be called, our connector will be long gone. Therefore, the
> > >>  connector must be cleaned up when the bridge is detached to avoid
> > >>  use-after-free conditions.
> > >
> > > For -fixes this sounds ok, but for -next I think switching to drmm_
> > > would be much better.
> >
> > The API would need to change to have access to the drm_device struct,
> > though. That would be quite a big patch, there are a few dozens source
> > files that use this API already.
> 
> Hm right pure drmm_ doesn't work for panel or bridge since it's
> usually a separate driver. But devm_ also doesn't work. I think what
> we need here is two-stage: first kmalloc the panel (or bridge, it's
> really the same) in the panel/bridge driver load. Then when we bind it
> to the drm_device we can tie it into the managed resources with
> drmm_add_action_or_reset. Passing the drm_device to the point where we
> allocate the panel/bridge doesn't work for these.
> 
> I think minimally we need a FIXME here and ack from Laurent on how
> this should be solved at least, since panel bridge is used rather
> widely.

Bridge removal is completely broken. If you unbind a bridge driver from
the device, the bridge will be unregistered and resources freed, without
the display driver knowing about this. The lifetime of the drm_bridge
structure itself isn't the only issue to be addressed here, it's broader
than that, and needs to consider that the display driver could be
calling the bridge operations concurrently to the removal.

We need a volunteer with enough motivation to solve this subsystem-wide
:-) In the meantime, whatever shortcut addresses immediate issues is
probably fine, as yak-shaving in this area would definitely not be
reasonable.

> > >> v2: Cleanup connector only if it was created
> > >>
> > >> Fixes: 13dfc0540a57 ("drm/bridge: Refactor out the panel wrapper from the lvds-encoder bridge.")
> > >> Cc: <stable@vger.kernel.org> # 4.12+
> > >> Cc: Andrzej Hajda <a.hajda@samsung.com>
> > >> Cc: Neil Armstrong <narmstrong@baylibre.com>
> > >> Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
> > >> Cc: Jonas Karlman <jonas@kwiboo.se>
> > >> Cc: Jernej Skrabec <jernej.skrabec@siol.net>
> > >> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> > >> ---
> > >>  drivers/gpu/drm/bridge/panel.c | 6 ++++++
> > >>  1 file changed, 6 insertions(+)
> > >>
> > >> diff --git a/drivers/gpu/drm/bridge/panel.c b/drivers/gpu/drm/bridge/panel.c
> > >> index 0ddc37551194..df86b0ee0549 100644
> > >> --- a/drivers/gpu/drm/bridge/panel.c
> > >> +++ b/drivers/gpu/drm/bridge/panel.c
> > >> @@ -87,6 +87,12 @@ static int panel_bridge_attach(struct drm_bridge *bridge,
> > >>
> > >>  static void panel_bridge_detach(struct drm_bridge *bridge)
> > >>  {
> > >> +	struct panel_bridge *panel_bridge = drm_bridge_to_panel_bridge(bridge);
> > >> +	struct drm_connector *connector = &panel_bridge->connector;
> > >> +
> > >> +	/* Cleanup the connector if we know it was initialized */
> > >> +	if (!!panel_bridge->connector.dev)
> > >> +		drm_connector_cleanup(connector);
> > >>  }
> > >>
> > >>  static void panel_bridge_pre_enable(struct drm_bridge *bridge)

-- 
Regards,

Laurent Pinchart