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

[Richard Chang <richardycc@google.com>]

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: f405066a1f0d ("zram: introduce writeback bio batching")
Signed-off-by: Richard Chang <richardycc@google.com>
---
 drivers/block/zram/zram_drv.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index aebc710f0d6a..a457fdf564f8 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -966,9 +966,8 @@ static void zram_writeback_endio(struct bio *bio)
 
 	spin_lock_irqsave(&wb_ctl->done_lock, flags);
 	list_add(&req->entry, &wb_ctl->done_reqs);
-	spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
-
 	wake_up(&wb_ctl->done_wait);
+	spin_unlock_irqrestore(&wb_ctl->done_lock, flags);
 }
 
 static void zram_submit_wb_request(struct zram *zram,
-- 
2.54.0.545.g6539524ca2-goog