zram: fix use-after-free in zram_writeback_endio#795
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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 (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 (&wb_ctl->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(&wb_ctl->num_inflight);
returns to writeback_store();
// frees wb_ctl
release_wb_ctl(wb_ctl);
// UAF crash!
wake_up(&wb_ctl->done_wait);
Fix this by moving wake_up() inside the done_lock critical section.
This ensures that zram_complete_done_reqs cannot consume the request
and decrement num_inflight until zram_writeback_endio has finished
calling wake_up() and released the lock.
Fixes: f405066 ("zram: introduce writeback bio batching")
Signed-off-by: Richard Chang <richardycc@google.com>
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
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Pull request for series with
subject: zram: fix use-after-free in zram_writeback_endio
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1089254