[PATCH] drm: damage_helper: Fix race checking plane->state->fb

[Sean Paul <sean@poorly.run>]

From: Sean Paul <seanpaul@chromium.org>

Since the dirtyfb ioctl doesn't give us any hints as to which plane is
scanning out the fb it's marking as damaged, we need to loop through
planes to find it.

Currently we just reach into plane state and check, but that can race
with another commit changing the fb out from under us. This patch locks
the plane before checking the fb and will release the lock if the plane
is not displaying the dirty fb.

Fixes: b9fc5e01d1ce ("drm: Add helper to implement legacy dirtyfb")
Cc: Rob Clark <robdclark@gmail.com>
Cc: Deepak Rawat <drawat@vmware.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.0+
Reported-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
---
 drivers/gpu/drm/drm_damage_helper.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_damage_helper.c b/drivers/gpu/drm/drm_damage_helper.c
index 8230dac01a89..3a4126dc2520 100644
--- a/drivers/gpu/drm/drm_damage_helper.c
+++ b/drivers/gpu/drm/drm_damage_helper.c
@@ -212,8 +212,14 @@ int drm_atomic_helper_dirtyfb(struct drm_framebuffer *fb,
 	drm_for_each_plane(plane, fb->dev) {
 		struct drm_plane_state *plane_state;
 
-		if (plane->state->fb != fb)
+		ret = drm_modeset_lock(&plane->mutex, state->acquire_ctx);
+		if (ret)
+			goto out;
+
+		if (plane->state->fb != fb) {
+			drm_modeset_unlock(&plane->mutex);
 			continue;
+		}
 
 		plane_state = drm_atomic_get_plane_state(state, plane);
 		if (IS_ERR(plane_state)) {
-- 
Sean Paul, Software Engineer, Google / Chromium OS