In the Linux kernel, the following vulnerability has been resolved: binder: fix UAF in binder_free_transaction() In binder_free_transactio

In the Linux kernel, the following vulnerability has been resolved:

binder: fix UAF in binder_free_transaction()

In binder_free_transaction(), the t->to_proc is read under the t->lock.
However, once the t->lock is dropped, the to_proc can die in parallel.
This leads to a use-after-free error when we attempt to acquire its
inner lock right afterwards:

==================================================================
BUG: KASAN: slab-use-after-free in _raw_spin_lock+0xe4/0x1a0
Write of size 4 at addr ffff00001125da70 by task B/672

CPU: 20 UID: 0 PID: 672 Comm: B Not tainted 7.1.0-rc6-00284-g8e65320d91cd #4 PREEMPT
Hardware name: linux,dummy-virt (DT)
Call trace:
_raw_spin_lock+0xe4/0x1a0
binder_free_transaction+0x8c/0x320
binder_send_failed_reply+0x21c/0x2f8
binder_thread_release+0x488/0x7e0
binder_ioctl+0x12c0/0x29a0
[...]

Allocated by task 675:
__kmalloc_cache_noprof+0x174/0x444
binder_open+0x118/0xb70
do_dentry_open+0x374/0x1040
vfs_open+0x58/0x3bc
[...]

Freed by task 212:
__kasan_slab_free+0x58/0x80
kfree+0x1a0/0x4a4
binder_proc_dec_tmpref+0x32c/0x5e0
binder_deferred_func+0xc48/0x104c
process_one_work+0x53c/0xbc0
[...]

To prevent this, pin the target thread (t->to_thread) to guarantee the
target process remains alive. Undelivered transactions without a target
thread are already safe, as the target process can only be the current
context in those paths.