From: Zizhi Wo In null_init(), configfs_register_subsystem() currently runs before register_blkdev(), so when null_blk is built as a module, a racing mkdir() + poweron from userspace can reach null_add_dev() while null_major is still 0. __add_disk() then hits WARN_ON(disk->minors) (major=0 with minors!=0) and fails: [root@fedora ~]# [ 2366.521436] WARNING: block/genhd.c:476 at __add_disk+0x8a7/0xde0, [ 2366.523552] Modules linked in: null_blk(+) nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib [ 2366.529081] CPU: 26 UID: 0 PID: 1600 Comm: sh Not tainted 7.2.0-rc1+ #66 PREEMPT(full) ...... [ 2366.547251] Call Trace: [ 2366.547575] [ 2366.547831] ? _raw_spin_lock+0x84/0xe0 [ 2366.548260] add_disk_fwnode+0x114/0x560 [ 2366.548739] null_add_dev+0x102d/0x1b80 [null_blk] [ 2366.549310] ? __pfx_null_add_dev+0x10/0x10 [null_blk] [ 2366.549906] ? mutex_lock+0xde/0x1c0 [ 2366.550361] ? __pfx_mutex_lock+0x10/0x10 [ 2366.550827] nullb_device_power_store+0x1e7/0x280 [null_blk] [ 2366.551499] ? __pfx_nullb_device_power_store+0x10/0x10 [null_blk] [ 2366.552177] ? __kmalloc_cache_noprof+0x1f5/0x470 [ 2366.552748] ? configfs_write_iter+0x35c/0x4e0 [ 2366.553242] configfs_write_iter+0x286/0x4e0 [ 2366.553787] vfs_write+0x52d/0xd00 [ 2366.554169] ? __pfx_vfs_write+0x10/0x10 [ 2366.554679] ? __pfx___css_rstat_updated+0x10/0x10 [ 2366.555196] ? fdget_pos+0x1cf/0x4c0 [ 2366.555649] ksys_write+0xfc/0x1d0 ...... Additionally, the err_dev path destroys all devices on nullb_list while configfs is still registered. If a racing mkdir() + poweron puts a user device on the list, null_destroy_dev()->null_free_dev() kfrees the user device's nullb_device but /sys/kernel/config/nullb/ is still reachable. Any userspace access to the item will trigger a UAF. For simplicity, move configfs_register_subsystem() to the end to solve the problems above. Fixes: 3bf2bd20734e ("nullb: add configfs interface") Signed-off-by: Zizhi Wo Reviewed-by: Damien Le Moal --- drivers/block/null_blk/main.c | 16 ++++++---------- 1 file changed, 6 insertions(+), 10 deletions(-) diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c index eba204b27785..4613035222cd 100644 --- a/drivers/block/null_blk/main.c +++ b/drivers/block/null_blk/main.c @@ -2162,15 +2162,9 @@ static int __init null_init(void) config_group_init(&nullb_subsys.su_group); mutex_init(&nullb_subsys.su_mutex); - ret = configfs_register_subsystem(&nullb_subsys); - if (ret) - return ret; - null_major = register_blkdev(0, "nullb"); - if (null_major < 0) { - ret = null_major; - goto err_conf; - } + if (null_major < 0) + return null_major; for (i = 0; i < nr_devices; i++) { ret = null_create_dev(); @@ -2178,6 +2172,10 @@ static int __init null_init(void) goto err_dev; } + ret = configfs_register_subsystem(&nullb_subsys); + if (ret) + goto err_dev; + pr_info("module loaded\n"); return 0; @@ -2187,8 +2185,6 @@ static int __init null_init(void) null_destroy_dev(nullb); } unregister_blkdev(null_major, "nullb"); -err_conf: - configfs_unregister_subsystem(&nullb_subsys); return ret; } -- 2.52.0