Re: [Intel-gfx] [PATCH v2] drm/i915: Fix vm use-after-free in vma destruction

["Das, Nirmoy" <nirmoy.das@linux.intel.com>]

Acked-by: Nirmoy Das <nirmoy.das@intel.con>

On 6/20/2022 2:36 PM, Thomas Hellström wrote:
> In vma destruction, the following race may occur:
>
> Thread 1:	    		  Thread 2:
> i915_vma_destroy();
>
>    ...
>    list_del_init(vma->vm_link);
>    ...
>    mutex_unlock(vma->vm->mutex);
> 				  __i915_vm_release();
> release_references();
>
> And in release_reference() we dereference vma->vm to get to the
> vm gt pointer, leading to a use-after free.
>
> However, __i915_vm_release() grabs the vm->mutex so the vm won't be
> destroyed before vma->vm->mutex is released, so extract the gt pointer
> under the vm->mutex to avoid the vma->vm dereference in
> release_references().
>
> v2: Fix a typo in the commit message (Andi Shyti)
>
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/5944
> Fixes: e1a7ab4fca ("drm/i915: Remove the vm open count")
>
> Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
> Cc: Matthew Auld <matthew.auld@intel.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>   drivers/gpu/drm/i915/i915_vma.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> index 0bffb70b3c5f..04d12f278f57 100644
> --- a/drivers/gpu/drm/i915/i915_vma.c
> +++ b/drivers/gpu/drm/i915/i915_vma.c
> @@ -1637,10 +1637,10 @@ static void force_unbind(struct i915_vma *vma)
>   	GEM_BUG_ON(drm_mm_node_allocated(&vma->node));
>   }
>   
> -static void release_references(struct i915_vma *vma, bool vm_ddestroy)
> +static void release_references(struct i915_vma *vma, struct intel_gt *gt,
> +			       bool vm_ddestroy)
>   {
>   	struct drm_i915_gem_object *obj = vma->obj;
> -	struct intel_gt *gt = vma->vm->gt;
>   
>   	GEM_BUG_ON(i915_vma_is_active(vma));
>   
> @@ -1695,11 +1695,12 @@ void i915_vma_destroy_locked(struct i915_vma *vma)
>   
>   	force_unbind(vma);
>   	list_del_init(&vma->vm_link);
> -	release_references(vma, false);
> +	release_references(vma, vma->vm->gt, false);
>   }
>   
>   void i915_vma_destroy(struct i915_vma *vma)
>   {
> +	struct intel_gt *gt;
>   	bool vm_ddestroy;
>   
>   	mutex_lock(&vma->vm->mutex);
> @@ -1707,8 +1708,11 @@ void i915_vma_destroy(struct i915_vma *vma)
>   	list_del_init(&vma->vm_link);
>   	vm_ddestroy = vma->vm_ddestroy;
>   	vma->vm_ddestroy = false;
> +
> +	/* vma->vm may be freed when releasing vma->vm->mutex. */
> +	gt = vma->vm->gt;
>   	mutex_unlock(&vma->vm->mutex);
> -	release_references(vma, vm_ddestroy);
> +	release_references(vma, gt, vm_ddestroy);
>   }
>   
>   void i915_vma_parked(struct intel_gt *gt)