Re: [PATCH v2] zram: fix use-after-free in zram_writeback_endio

[Minchan Kim <minchan@kernel.org>]

On Fri, May 08, 2026 at 08:49:33AM +0000, Richard Chang wrote:
> 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.
> 
> Fixes: f405066a1f0d ("zram: introduce writeback bio batching")
> Suggested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> Suggested-by: Minchan Kim <minchan@kernel.org>
> Signed-off-by: Richard Chang <richardycc@google.com>
Acked-by: Minchan Kim <minchan@kernel.org>