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")
Cc: stable@vger.kernel.org
Suggested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Suggested-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Richard Chang <richardycc@google.com>
---
V2: use RCU to manage the wb_ctl lifetime
V3: add stable tag

 drivers/block/zram/zram_drv.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index aebc710f0d6a..07111455eecf 100644
--- 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,
-- 
2.54.0.563.g4f69b47b94-goog

>@@ -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);
> }

Do we need to add a 'rcu_assign_pointer(wb_ctl, NULL);' before 'kfree_rcu(wb_ctl, rcu)'?

Signed-off-by: wang wei <a929244872@163.com>

-- 
2.25.1

On Wed, May 13, 2026 at 10:02:18PM +0800, wang wei wrote:
> >@@ -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);
> > }
> 
> Do we need to add a 'rcu_assign_pointer(wb_ctl, NULL);' before 'kfree_rcu(wb_ctl, rcu)'?
> 
> Signed-off-by: wang wei <a929244872@163.com>

Why do we need it?

My understanding is rcu_assign_pointer() is typically used to publish NULL to
a shared pointer variable so that future RCU readers (using rcu_dereference)
won't access the object before kfree_rcu().

However, in our case, wb_ctl is not stored in any shared pointer variable.
It is a local variable in writeback_store() and RCU readers (zram_writeback_endio)
do not look up wb_ctl from a shared pointer. They obtain it directly from
bio->bi_private of the specific bio they are completing.

Please let me know if I missed anything.