[PATCH] drm/i915: Fix potential bit_17 double-free

[Rob Clark]

From: Rob Clark <robdclark at chromium.org>

A userspace with multiple threads racing I915_GEM_SET_TILING to set the
tiling to I915_TILING_NONE could trigger a double free of the bit_17
bitmask.  (Or conversely leak memory on the transition to tiled.)  Move
allocation/free'ing of the bitmask within the section protected by the
obj lock.

Fixes: e9b73c67390a ("drm/i915: Reduce memory pressure during shrinker by preallocating swizzle pages")
Signed-off-by: Rob Clark <robdclark at chromium.org>
---
 drivers/gpu/drm/i915/gem/i915_gem_tiling.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_tiling.c b/drivers/gpu/drm/i915/gem/i915_gem_tiling.c
index fd42b89b7162..bc21b1c2350a 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_tiling.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_tiling.c
@@ -298,36 +298,37 @@ i915_gem_object_set_tiling(struct drm_i915_gem_object *obj,
 		vma->fence_alignment =
 			i915_gem_fence_alignment(i915,
 						 vma->size, tiling, stride);
 
 		if (vma->fence)
 			vma->fence->dirty = true;
 	}
 	spin_unlock(&obj->vma.lock);
 
 	obj->tiling_and_stride = tiling | stride;
-	i915_gem_object_unlock(obj);
-
-	/* Force the fence to be reacquired for GTT access */
-	i915_gem_object_release_mmap_gtt(obj);
 
 	/* Try to preallocate memory required to save swizzling on put-pages */
 	if (i915_gem_object_needs_bit17_swizzle(obj)) {
 		if (!obj->bit_17) {
 			obj->bit_17 = bitmap_zalloc(obj->base.size >> PAGE_SHIFT,
 						    GFP_KERNEL);
 		}
 	} else {
 		bitmap_free(obj->bit_17);
 		obj->bit_17 = NULL;
 	}
 
+	i915_gem_object_unlock(obj);
+
+	/* Force the fence to be reacquired for GTT access */
+	i915_gem_object_release_mmap_gtt(obj);
+
 	return 0;
 }
 
 /**
  * i915_gem_set_tiling_ioctl - IOCTL handler to set tiling mode
  * @dev: DRM device
  * @data: data pointer for the ioctl
  * @file: DRM file for the ioctl call
  *
  * Sets the tiling mode of an object, returning the required swizzling of
-- 
2.38.1