From: Chenwandun chenwandun@huawei.com Subject: zram: fix race between backing_dev_show and backing_dev_store

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);

gets the value of zram->backing_dev too early in backing_dev_show, which resultin the value being NULL at the beginning, 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

Link: http://lkml.kernel.org/r/1571046839-16814-1-git-send-email-chenwandun@huawei... Signed-off-by: Chenwandun chenwandun@huawei.com Acked-by: Minchan Kim minchan@kernel.org Cc: Sergey Senozhatsky sergey.senozhatsky.work@gmail.com Cc: Jens Axboe axboe@kernel.dk Cc: stable@vger.kernel.org [4.14+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

drivers/block/zram/zram_drv.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/drivers/block/zram/zram_drv.c~zram-fix-race-between-backing_dev_show-and-backing_dev_store +++ a/drivers/block/zram/zram_drv.c @@ -413,13 +413,14 @@ static void reset_bdev(struct zram *zram static ssize_t backing_dev_show(struct device *dev, struct device_attribute *attr, char *buf) { + struct file *file; struct zram *zram = dev_to_zram(dev); - struct file *file = zram->backing_dev; char *p; ssize_t ret;

down_read(&zram->init_lock); - if (!zram->backing_dev) { + file = zram->backing_dev; + if (!file) { memcpy(buf, "none\n", 5); up_read(&zram->init_lock); return 5; _