[PATCH] binder: fix UAF when releasing todo list

Christian Brauner christian.brauner at ubuntu.com
Tue Oct 13 20:08:23 UTC 2020


On Fri, Oct 09, 2020 at 04:24:55PM -0700, Todd Kjos 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>
> ---

Thanks!
Acked-by: Christian Brauner <christian.brauner at ubuntu.com>


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