zram: fix use-after-free in zram_writeback_endio

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

zram: fix use-after-free in zram_writeback_endio

A crash was observed in zram_writeback_endio due to a NULL pointer

dereference in wake_up. The root cause is a race condition between the

bio completion handler (zram_writeback_endio) and the writeback task.

In zram_writeback_endio, wake_up() is called on &wb_ctl->done_wait after

releasing wb_ctl->done_lock. This creates a race window where the

writeback task can see num_inflight become 0, return, and free wb_ctl

before zram_writeback_endio calls wake_up().

CPU 0 (zram_writeback_endio) CPU 1 (writeback_store)

============================ ============================

zram_writeback_slots

zram_submit_wb_request

zram_submit_wb_request

wait_event(wb_ctl->done_wait)

spin_lock(&wb_ctl->done_lock);

list_add(&req->entry, &wb_ctl->done_reqs);

spin_unlock(&wb_ctl->done_lock);

wake_up(&wb_ctl->done_wait);

zram_complete_done_reqs

spin_lock(&wb_ctl->done_lock);

list_add(&req->entry, &wb_ctl->done_reqs);

spin_unlock(&wb_ctl->done_lock);

while (num_inflight) > 0)

spin_lock(&wb_ctl->done_lock);

list_del(&req->entry);

spin_unlock(&wb_ctl->done_lock);

// num_inflight becomes 0

atomic_dec(num_inflight);

// Leave zram_writeback_slots

// Free wb_ctl

release_wb_ctl(wb_ctl);

// UAF crash!

wake_up(&wb_ctl->done_wait);

This patch fixes this race by using RCU. By protecting wb_ctl with

rcu_read_lock() in zram_writeback_endio and using kfree_rcu() to free it,

we ensure that wb_ctl remains valid during the execution of

zram_writeback_endio.