tree 03d23b31ae7fde894df1bf0ba1164b0cbb237700
parent 007258f99b02f924f04f13929f840af4c521f983
author Todd Kjos <tkjos@google.com> 1595304873 -0700
committer Luca Weiss <luca.weiss@fairphone.com> 1626347891 +0200

binder: fix UAF when releasing todo list

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() calls 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.

Bug: 161151868
Signed-off-by: Todd Kjos <tkjos@google.com>
Change-Id: I4ae752abfe1aa38872be6f266ddd271802952625