[PATCH 4.19 014/264] binder: fix UAF when releasing todo list

Greg Kroah-Hartman
2020-10-27T18:04:31Z

From: Todd Kjos <tkjos@google.com>

commit f3277cbfba763cd2826396521b9296de67cf1bbc upstream.

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@google.com>
Link: https://lore.kernel.org/r/20201009232455.4054810-1-tkjos@google.com
Cc: <stable@vger.kernel.org> # 4.14, 4.19, 5.4, 5.8
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/android/binder.c | 35 ++++++++++-------------------------
1 file changed, 10 insertions(+), 25 deletions(-)

--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -285,7 +285,7 @@ struct binder_device {
struct binder_work {
struct list_head entry;

- enum {
+ enum binder_work_type {
BINDER_WORK_TRANSACTION = 1,
BINDER_WORK_TRANSACTION_COMPLETE,
BINDER_WORK_RETURN_ERROR,
@@ -895,27 +895,6 @@ static struct binder_work *binder_dequeu
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);
@@ -4242,13 +4221,17 @@ static void binder_release_work(struct b
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;

@@ -4282,9 +4265,11 @@ static void binder_release_work(struct b
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;
}
}