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

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On 27/01/2023 20:05, Rob Clark wrote:
From: Rob Clark <robdclark@xxxxxxxxxxxx>

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")

Should be:

Fixes: 2850748ef876 ("drm/i915: Pull i915_vma_pin under the vm->mutex")

Before that commit there as a "BKL" (struct_mutex) aroung the call to i915_gem_object_set_tiling. Otherwise fix looks good:

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

I'll tweak the fixes tag and merge in a minute, thanks for the fix!

Regards,

Tvrtko

Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
---
  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



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