Re: [PATCH] zram: fix race between backing_dev_show and backing_dev_store

[Minchan Kim <minchan@kernel.org>]

On Mon, Oct 14, 2019 at 05:53:59PM +0800, Chen Wandun wrote:
> From: Chenwandun <chenwandun@huawei.com>
> 
> CPU0:				       CPU1:
> backing_dev_show		       backing_dev_store
>     ......				   ......
>     file = zram->backing_dev;
>     down_read(&zram->init_lock);	   down_read(&zram->init_init_lock)
>     file_path(file, ...);		   zram->backing_dev = backing_dev;
>     up_read(&zram->init_lock);		   up_read(&zram->init_lock);
> 
> get the value of zram->backing_dev too early in backing_dev_show,
> that will result the value may be NULL at the begining, and not
> NULL later.
> 
> backtrace:
> [<ffffff8570e0f3ec>] d_path+0xcc/0x174
> [<ffffff8570decd90>] file_path+0x10/0x18
> [<ffffff85712f7630>] backing_dev_show+0x40/0xb4
> [<ffffff85712c776c>] dev_attr_show+0x20/0x54
> [<ffffff8570e835e4>] sysfs_kf_seq_show+0x9c/0x10c
> [<ffffff8570e82b98>] kernfs_seq_show+0x28/0x30
> [<ffffff8570e1c580>] seq_read+0x184/0x488
> [<ffffff8570e81ec4>] kernfs_fop_read+0x5c/0x1a4
> [<ffffff8570dee0fc>] __vfs_read+0x44/0x128
> [<ffffff8570dee310>] vfs_read+0xa0/0x138
> [<ffffff8570dee860>] SyS_read+0x54/0xb4
> 
> Signed-off-by: Chenwandun <chenwandun@huawei.com>

It should be stable material.
Cc: <stable@vger.kernel.org> [4.14+]
Acked-by: Minchan Kim <minchan@kernel.org>

Thanks!