[PATCH] drm/ttm: fix use-after-free races in vm fault handling

[Christian König]

Am 18.02.2017 um 23:50 schrieb Nicolai Hähnle:
> From: Nicolai Hähnle <nicolai.haehnle at amd.com>
>
> The vm fault handler relies on the fact that the VMA owns a reference
> to the BO. However, once mmap_sem is released, other tasks are free to
> destroy the VMA, which can lead to the BO being freed. Fix two code
> paths where that can happen, both related to vm fault retries.
>
> Found via a lock debugging warning which flagged &bo->wu_mutex as
> locked while being destroyed.
>
> Fixes: cbe12e74ee4e ("drm/ttm: Allow vm fault retries")
> Signed-off-by: Nicolai Hähnle <nicolai.haehnle at amd.com>

Good catch! Patch is Reviewed-by: Christian König <christian.koenig at amd.com>

> --
> This does not fix the random memory corruption I've been seeing.
> ---
>   drivers/gpu/drm/ttm/ttm_bo_vm.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index a6ed9d5..750733a 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -66,8 +66,11 @@ static int ttm_bo_vm_fault_idle(struct ttm_buffer_object *bo,
>   		if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
>   			goto out_unlock;
>   
> +		ttm_bo_reference(bo);
>   		up_read(&vma->vm_mm->mmap_sem);
>   		(void) fence_wait(bo->moving, true);
> +		ttm_bo_unreserve(bo);
> +		ttm_bo_unref(&bo);
>   		goto out_unlock;
>   	}
>   
> @@ -120,8 +123,10 @@ static int ttm_bo_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
>   
>   		if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
>   			if (!(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
> +				ttm_bo_reference(bo);
>   				up_read(&vma->vm_mm->mmap_sem);
>   				(void) ttm_bo_wait_unreserved(bo);
> +				ttm_bo_unref(&bo);
>   			}
>   
>   			return VM_FAULT_RETRY;
> @@ -166,6 +171,13 @@ static int ttm_bo_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
>   	ret = ttm_bo_vm_fault_idle(bo, vma, vmf);
>   	if (unlikely(ret != 0)) {
>   		retval = ret;
> +
> +		if (retval == VM_FAULT_RETRY &&
> +		    !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
> +			/* The BO has already been unreserved. */
> +			return retval;
> +		}
> +
>   		goto out_unlock;
>   	}
>