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

[Tvrtko Ursulin]

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