From 552b1f893006450e4466b651e4e978b076b5dd96 Mon Sep 17 00:00:00 2001 From: Richard Chang Date: Mon, 4 May 2026 12:32:30 +0000 Subject: [PATCH] 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 (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 Acked-by: Sergey Senozhatsky --- 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,