Re: [PATCH v6] f2fs: fix use-after-free in f2fs_write_end_io

[Chao Yu <chao@kernel.org>]

On 1/6/2026 9:06 PM, Szymon Wilczek wrote:
> Syzbot reported a slab-use-after-free issue in f2fs_write_end_io():
> 
> BUG: KASAN: slab-use-after-free in f2fs_write_end_io+0x9b9/0xb60
> Read of size 4 at addr ffff88804357d170 by task kworker/u4:4/45
> 
> The race condition occurs between the filesystem unmount path
> (kill_f2fs_super) and the asynchronous I/O completion handler
> (f2fs_write_end_io).
> 
> When unmounting, kill_f2fs_super() frees the sbi structure. However,
> if the bio completion callback f2fs_write_end_io() is still running
> in softirq context, it may access sbi->cp_wait after sbi has been
> freed, causing a use-after-free.
> 
> Fix this by calling synchronize_rcu() before kfree(sbi). Since
> bio completion callbacks run in softirq context, which is an implicit
> RCU read-side critical section, synchronize_rcu() ensures all
> in-flight callbacks have completed before we free sbi.
> 
> Reported-by: syzbot+b4444e3c972a7a124187@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=b4444e3c972a7a124187

As I checked w/ reproduer in syzbot, the patch doesn't fix the UAF issue.

https://lore.kernel.org/lkml/695e0473.050a0220.1c677c.0356.GAE@google.com

Thanks,

> Signed-off-by: Szymon Wilczek <swilczek.lx@gmail.com>
> ---
> v6: Add comment to explain synchronize_rcu() call.
>      Resend as reply to original thread.
> v5: Resend as reply to original thread (v4 was mistakenly sent to a new thread).
> v4: Removed f2fs_wait_on_all_pages() call as pointed out by Chao Yu that
>      it accesses sbi->write_io which has already been freed in f2fs_put_super().
> v3: Simplified to minimal fix - only super.c change with synchronize_rcu(),
>      as pointed out by Chao Yu that data.c changes are not necessary since
>      synchronize_rcu() alone guarantees sbi won't be freed before callbacks
>      complete.
> v2: Add synchronize_rcu() to wait for softirq bio callbacks to complete.
>   fs/f2fs/super.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
> index c4c225e09dc4..f5707591ba25 100644
> --- a/fs/f2fs/super.c
> +++ b/fs/f2fs/super.c
> @@ -5454,6 +5454,8 @@ static void kill_f2fs_super(struct super_block *sb)
>   	kill_block_super(sb);
>   	/* Release block devices last, after fscrypt_destroy_keyring(). */
>   	if (sbi) {
> +		/* wait for f2fs_write_end_io() to finish */
> +		synchronize_rcu();
>   		destroy_device_list(sbi);
>   		kfree(sbi);
>   		sb->s_fs_info = NULL;