[Intel-gfx] [PATCH] drm/i915: Fix null pointer dereference in ring cleanup code

[John.C.Harrison at Intel.com]

From: John Harrison <John.C.Harrison at Intel.com>

If a ring failed to initialise for any reason then the error path would try to
clean up all rings including those that had not yet been allocated. The ring
clean up code did a check that the ring was valid before starting its work.
Unfortunately, that was after it had already dereferenced the ring to obtain a
dev_private pointer.

Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
---
 drivers/gpu/drm/i915/intel_lrc.c        |    4 +++-
 drivers/gpu/drm/i915/intel_ringbuffer.c |    7 +++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index cd74e5c..76776fa 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -1214,11 +1214,13 @@ static int gen8_emit_request(struct intel_ringbuffer *ringbuf)
  */
 void intel_logical_ring_cleanup(struct intel_engine_cs *ring)
 {
-	struct drm_i915_private *dev_priv = ring->dev->dev_private;
+	struct drm_i915_private *dev_priv;
 
 	if (!intel_ring_initialized(ring))
 		return;
 
+	dev_priv = ring->dev->dev_private;
+
 	intel_logical_ring_stop(ring);
 	WARN_ON((I915_READ_MODE(ring) & MODE_IDLE) == 0);
 	ring->preallocated_lazy_request = NULL;
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index a8f72e8..f457146 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1845,12 +1845,15 @@ error:
 
 void intel_cleanup_ring_buffer(struct intel_engine_cs *ring)
 {
-	struct drm_i915_private *dev_priv = to_i915(ring->dev);
-	struct intel_ringbuffer *ringbuf = ring->buffer;
+	struct drm_i915_private *dev_priv;
+	struct intel_ringbuffer *ringbuf;
 
 	if (!intel_ring_initialized(ring))
 		return;
 
+	dev_priv = to_i915(ring->dev);
+	ringbuf = ring->buffer;
+
 	intel_stop_ring_buffer(ring);
 	WARN_ON(!IS_GEN2(ring->dev) && (I915_READ_MODE(ring) & MODE_IDLE) == 0);
 
-- 
1.7.9.5