[PATCH] binder: fix UAF when releasing todo list

[Todd Kjos]

On Fri, Oct 9, 2020 at 4:24 PM Todd Kjos <tkjos at google.com> wrote:
>
> When releasing a thread todo list when tearing down
> a binder_proc, the following race was possible which
> could result in a use-after-free:
>
> 1.  Thread 1: enter binder_release_work from binder_thread_release
> 2.  Thread 2: binder_update_ref_for_handle() -> binder_dec_node_ilocked()
> 3.  Thread 2: dec nodeA --> 0 (will free node)
> 4.  Thread 1: ACQ inner_proc_lock
> 5.  Thread 2: block on inner_proc_lock
> 6.  Thread 1: dequeue work (BINDER_WORK_NODE, part of nodeA)
> 7.  Thread 1: REL inner_proc_lock
> 8.  Thread 2: ACQ inner_proc_lock
> 9.  Thread 2: todo list cleanup, but work was already dequeued
> 10. Thread 2: free node
> 11. Thread 2: REL inner_proc_lock
> 12. Thread 1: deref w->type (UAF)
>
> The problem was that for a BINDER_WORK_NODE, the binder_work element
> must not be accessed after releasing the inner_proc_lock while
> processing the todo list elements since another thread might be
> handling a deref on the node containing the binder_work element
> leading to the node being freed.
>
> Signed-off-by: Todd Kjos <tkjos at google.com>

Cc: <stable at vger.kernel.org> # 4.14, 4.19, 5.4, 5.8

> ---
>  drivers/android/binder.c | 35 ++++++++++-------------------------
>  1 file changed, 10 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index f936530a19b0..e8a1db4a86ed 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -223,7 +223,7 @@ static struct binder_transaction_log_entry *binder_transaction_log_add(
>  struct binder_work {
>         struct list_head entry;
>
> -       enum {
> +       enum binder_work_type {
>                 BINDER_WORK_TRANSACTION = 1,
>                 BINDER_WORK_TRANSACTION_COMPLETE,
>                 BINDER_WORK_RETURN_ERROR,
> @@ -885,27 +885,6 @@ static struct binder_work *binder_dequeue_work_head_ilocked(
>         return w;
>  }
>
> -/**
> - * binder_dequeue_work_head() - Dequeues the item at head of list
> - * @proc:         binder_proc associated with list
> - * @list:         list to dequeue head
> - *
> - * Removes the head of the list if there are items on the list
> - *
> - * Return: pointer dequeued binder_work, NULL if list was empty
> - */
> -static struct binder_work *binder_dequeue_work_head(
> -                                       struct binder_proc *proc,
> -                                       struct list_head *list)
> -{
> -       struct binder_work *w;
> -
> -       binder_inner_proc_lock(proc);
> -       w = binder_dequeue_work_head_ilocked(list);
> -       binder_inner_proc_unlock(proc);
> -       return w;
> -}
> -
>  static void
>  binder_defer_work(struct binder_proc *proc, enum binder_deferred_state defer);
>  static void binder_free_thread(struct binder_thread *thread);
> @@ -4587,13 +4566,17 @@ static void binder_release_work(struct binder_proc *proc,
>                                 struct list_head *list)
>  {
>         struct binder_work *w;
> +       enum binder_work_type wtype;
>
>         while (1) {
> -               w = binder_dequeue_work_head(proc, list);
> +               binder_inner_proc_lock(proc);
> +               w = binder_dequeue_work_head_ilocked(list);
> +               wtype = w ? w->type : 0;
> +               binder_inner_proc_unlock(proc);
>                 if (!w)
>                         return;
>
> -               switch (w->type) {
> +               switch (wtype) {
>                 case BINDER_WORK_TRANSACTION: {
>                         struct binder_transaction *t;
>
> @@ -4627,9 +4610,11 @@ static void binder_release_work(struct binder_proc *proc,
>                         kfree(death);
>                         binder_stats_deleted(BINDER_STAT_DEATH);
>                 } break;
> +               case BINDER_WORK_NODE:
> +                       break;
>                 default:
>                         pr_err("unexpected work type, %d, not freed\n",
> -                              w->type);
> +                              wtype);
>                         break;
>                 }
>         }
> --
> 2.28.0.1011.ga647a8990f-goog
>