[PATCH 4.19 09/32] binder: fix possible UAF when freeing buffer

Greg Kroah-Hartman
2019-08-02T09:56:26Z

From: Todd Kjos <tkjos@android.com>

commit a370003cc301d4361bae20c9ef615f89bf8d1e8a upstream.

There is a race between the binder driver cleaning
up a completed transaction via binder_free_transaction()
and a user calling binder_ioctl(BC_FREE_BUFFER) to
release a buffer. It doesn't matter which is first but
they need to be protected against running concurrently
which can result in a UAF.

Signed-off-by: Todd Kjos <tkjos@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/android/binder.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -1960,8 +1960,18 @@ static struct binder_thread *binder_get_

static void binder_free_transaction(struct binder_transaction *t)
{
- if (t->buffer)
- t->buffer->transaction = NULL;
+ struct binder_proc *target_proc = t->to_proc;
+
+ if (target_proc) {
+ binder_inner_proc_lock(target_proc);
+ if (t->buffer)
+ t->buffer->transaction = NULL;
+ binder_inner_proc_unlock(target_proc);
+ }
+ /*
+ * If the transaction has no target_proc, then
+ * t->buffer->transaction has already been cleared.
+ */
kfree(t);
binder_stats_deleted(BINDER_STAT_TRANSACTION);
}
@@ -3484,10 +3494,12 @@ static int binder_thread_write(struct bi
buffer->debug_id,
buffer->transaction ? "active" : "finished");

+ binder_inner_proc_lock(proc);
if (buffer->transaction) {
buffer->transaction->buffer = NULL;
buffer->transaction = NULL;
}
+ binder_inner_proc_unlock(proc);
if (buffer->async_transaction && buffer->target_node) {
struct binder_node *buf_node;
struct binder_work *w;