drm/i915: Fix race on unreferencing the wrong mmio-flip-request

As we perform the mmio-flip without any locking and then try to acquire
the struct_mutex prior to dereferencing the request, it is possible for
userspace to queue a new pageflip before the worker can finish clearing
the old state - and then it will clear the new flip request. The result
is that the new flip could be completed before the GPU has finished
rendering.

The bugs stems from removing the seqno checking in
commit 536f5b5e86b225dab94c7ff8061ae482b6077387
Author: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Date:   Thu Nov 6 11:03:40 2014 +0200

    drm/i915: Make mmio flip wait for seqno in the work function

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 19 insertions, 20 deletions

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 3c8801cecd3f..f2ec1d6f9c6a 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -10793,22 +10793,18 @@ static void intel_do_mmio_flip(struct intel_crtc *intel_crtc)
 
 static void intel_mmio_flip_work_func(struct work_struct *work)
 {
-	struct intel_crtc *crtc =
-		container_of(work, struct intel_crtc, mmio_flip.work);
-	struct intel_mmio_flip *mmio_flip;
+	struct intel_mmio_flip *mmio_flip =
+		container_of(work, struct intel_mmio_flip, work);
 
-	mmio_flip = &crtc->mmio_flip;
-	if (mmio_flip->req)
-		WARN_ON(__i915_wait_request(mmio_flip->req,
-					    crtc->reset_counter,
-					    false, NULL, NULL) != 0);
+	if (mmio_flip->rq)
+		WARN_ON(__i915_wait_request(mmio_flip->rq,
+					    mmio_flip->crtc->reset_counter,
+					    false, NULL, NULL));
 
-	intel_do_mmio_flip(crtc);
-	if (mmio_flip->req) {
-		mutex_lock(&crtc->base.dev->struct_mutex);
-		i915_gem_request_assign(&mmio_flip->req, NULL);
-		mutex_unlock(&crtc->base.dev->struct_mutex);
-	}
+	intel_do_mmio_flip(mmio_flip->crtc);
+
+	i915_gem_request_unreference__unlocked(mmio_flip->rq);
+	kfree(mmio_flip);
 }
 
 static int intel_queue_mmio_flip(struct drm_device *dev,
@@ -10818,12 +10814,17 @@ static int intel_queue_mmio_flip(struct drm_device *dev,
 				 struct intel_engine_cs *ring,
 				 uint32_t flags)
 {
-	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
+	struct intel_mmio_flip *mmio_flip;
+
+	mmio_flip = kmalloc(sizeof(*mmio_flip), GFP_KERNEL);
+	if (mmio_flip == NULL)
+		return -ENOMEM;
 
-	i915_gem_request_assign(&intel_crtc->mmio_flip.req,
-				obj->last_write_req);
+	mmio_flip->rq = i915_gem_request_reference(obj->last_write_req);
+	mmio_flip->crtc = to_intel_crtc(crtc);
 
-	schedule_work(&intel_crtc->mmio_flip.work);
+	INIT_WORK(&mmio_flip->work, intel_mmio_flip_work_func);
+	schedule_work(&mmio_flip->work);
 
 	return 0;
 }
@@ -13564,8 +13565,6 @@ static void intel_crtc_init(struct drm_device *dev, int pipe)
 	dev_priv->plane_to_crtc_mapping[intel_crtc->plane] = &intel_crtc->base;
 	dev_priv->pipe_to_crtc_mapping[intel_crtc->pipe] = &intel_crtc->base;
 
-	INIT_WORK(&intel_crtc->mmio_flip.work, intel_mmio_flip_work_func);
-
 	drm_crtc_helper_add(&intel_crtc->base, &intel_helper_funcs);
 
 	WARN_ON(drm_crtc_index(&intel_crtc->base) != intel_crtc->pipe);