patch "binder: fix UAF when releasing todo list" added to char-misc-next

patch "binder: fix UAF when releasing todo list" added to char-misc-next

[gregkh]

This is a note to let you know that I've just added the patch titled

    binder: fix UAF when releasing todo list

to my char-misc git tree which can be found at
    git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
in the char-misc-next branch.

The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)

The patch will also be merged in the next major kernel release
during the merge window.

If you have any questions about this process, please let me know.


From f3277cbfba763cd2826396521b9296de67cf1bbc Mon Sep 17 00:00:00 2001
From: Todd Kjos <tkjos@google.com>
Date: Fri, 9 Oct 2020 16:24:55 -0700
Subject: 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() -> 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(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 49c0700816a5..4b9476521da6 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);
@@ -4585,13 +4564,17 @@ 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;
 
@@ -4625,9 +4608,11 @@ 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