Re: [PATCH v2] dma-buf: fix racing conflict of dma_heap_add()

[T.J. Mercier]

On Mon, Oct 31, 2022 at 7:53 AM Dawei Li <set_pte_at@xxxxxxxxxxx> wrote:
>
> Racing conflict could be:
> task A                 task B
> list_for_each_entry
> strcmp(h->name))
>                        list_for_each_entry
>                        strcmp(h->name)
> kzalloc                kzalloc
> ......                 .....
> device_create          device_create
> list_add
>                        list_add
>
> The root cause is that task B has no idea about the fact someone
> else(A) has inserted heap with same name when it calls list_add,
> so a potential collision occurs.
>
> v1: https://lore.kernel.org/all/TYCP286MB2323950197F60FC3473123B7CA349@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>
> v1->v2: Narrow down locking scope, check the existence of heap before
> insertion, as suggested by Andrew Davis.
>
> Fixes: c02a81fba74f ("dma-buf: Add dma-buf heaps framework")
>
> base-commit: 447fb14bf07905b880c9ed1ea92c53d6dd0649d7
>
> Signed-off-by: Dawei Li <set_pte_at@xxxxxxxxxxx>
> ---
>  drivers/dma-buf/dma-heap.c | 37 ++++++++++++++++++++++++++++---------
>  1 file changed, 28 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
> index 8f5848aa144f..1c787a147e3a 100644
> --- a/drivers/dma-buf/dma-heap.c
> +++ b/drivers/dma-buf/dma-heap.c
> @@ -216,9 +216,21 @@ const char *dma_heap_get_name(struct dma_heap *heap)
>         return heap->name;
>  }
>
> +static inline bool dma_heap_exist(const char *name)
> +{
> +       struct dma_heap *h;
> +
> +       list_for_each_entry(h, &heap_list, list) {
> +               if (!strcmp(h->name, name))
> +                       return true;
> +       }
> +
> +       return false;
> +}
> +
>  struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
>  {
> -       struct dma_heap *heap, *h, *err_ret;
> +       struct dma_heap *heap, *err_ret;
>         struct device *dev_ret;
>         unsigned int minor;
>         int ret;
> @@ -235,13 +247,11 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
>
>         /* check the name is unique */
>         mutex_lock(&heap_list_lock);
> -       list_for_each_entry(h, &heap_list, list) {
> -               if (!strcmp(h->name, exp_info->name)) {
> -                       mutex_unlock(&heap_list_lock);
> -                       pr_err("dma_heap: Already registered heap named %s\n",
> -                              exp_info->name);
> -                       return ERR_PTR(-EINVAL);
> -               }
> +       if (dma_heap_exist(exp_info->name)) {
> +               mutex_unlock(&heap_list_lock);
> +               pr_err("dma_heap: Already registered heap named %s\n",
> +                      exp_info->name);
> +               return ERR_PTR(-EINVAL);

Hi Dawei,
What Andrew was suggesting was that you entirely move the check from
here to the critical section down below, not duplicate the check. I
don't think we want to check this twice. We should be able to do this
by taking the heap_list_lock only once.

>         }
>         mutex_unlock(&heap_list_lock);
>
> @@ -283,13 +293,22 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
>                 err_ret = ERR_CAST(dev_ret);
>                 goto err2;
>         }
> +
>         /* Add heap to the list */
>         mutex_lock(&heap_list_lock);
> +       if (unlikely(dma_heap_exist(exp_info->name))) {
> +               mutex_unlock(&heap_list_lock);
> +               pr_err("dma_heap: Already registered heap named %s\n",
> +                      exp_info->name);
> +               err_ret = ERR_PTR(-EINVAL);
> +               goto err3;
> +       }
>         list_add(&heap->list, &heap_list);
>         mutex_unlock(&heap_list_lock);
>
>         return heap;
> -
> +err3:
> +       device_destroy(dma_heap_class, heap->heap_devt);
>  err2:
>         cdev_del(&heap->heap_cdev);
>  err1:
> --
> 2.25.1
>