Re: [PATCH V2 3/6] null_blk: register configfs subsystem after creating default devices

[Damien Le Moal]

On 7/7/26 11:55, Zizhi Wo wrote:
> From: Zizhi Wo <wozizhi@xxxxxxxxxx>
> 
> 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]  <TASK>
> [ 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/<name> 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. This also mirrors null_exit().
> 
> Fixes: 3bf2bd20734e ("nullb: add configfs interface")
> Signed-off-by: Zizhi Wo <wozizhi@xxxxxxxxxx>

Looks good.

Reviewed-by: Damien Le Moal <dlemoal@xxxxxxxxxx>

-- 
Damien Le Moal
Western Digital Research