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.

Link: https://lore.kernel.org/20260512074918.2606208-1-richardycc@google.com
Fixes: f405066a1f0d ("zram: introduce writeback bio batching")
Signed-off-by: Richard Chang <richardycc@google.com>
Suggested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Minchan Kim <minchan@kernel.org>
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Martin Liu <liumartin@google.com>
Cc: wang wei <a929244872@163.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index aebc710f0d6ae08adb0ab9b70146313800263ff2..07111455eecff7ab66b7e2b9c7da2f831b795fbb 100644 (file)
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -33,6 +33,7 @@
 #include <linux/cpuhotplug.h>
 #include <linux/part_stat.h>
 #include <linux/kernel_read_file.h>
+#include <linux/rcupdate.h>
 
 #include "zram_drv.h"
 
@@ -504,6 +505,7 @@ struct zram_wb_ctl {
        wait_queue_head_t done_wait;
        spinlock_t done_lock;
        atomic_t num_inflight;
+       struct rcu_head rcu;
 };
 
 struct zram_wb_req {
@@ -847,7 +849,7 @@ static void release_wb_ctl(struct zram_wb_ctl *wb_ctl)
                release_wb_req(req);
        }
 
-       kfree(wb_ctl);
+       kfree_rcu(wb_ctl, rcu);
 }
 
 static struct zram_wb_ctl *init_wb_ctl(struct zram *zram)
@@ -964,11 +966,13 @@ static void zram_writeback_endio(struct bio *bio)
        struct zram_wb_ctl *wb_ctl = bio->bi_private;
        unsigned long flags;
 
+       rcu_read_lock();
        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);
+       rcu_read_unlock();
 }
 
 static void zram_submit_wb_request(struct zram *zram,