published December 16, 2025
In the Linux kernel, the following vulnerability has been resolved:
blk-cgroup: fix possible deadlock while configuring policy
Following deadlock can be triggered easily by lockdep:
WARNING: possible circular locking dependency detected
6.17.0-rc3-00124-ga12c2658ced0 #1665 Not tainted
------------------------------------------------------
check/1334 is trying to acquire lock:
ff1100011d9d0678 (&q->sysfs_lock){+.+.}-{4:4}, at: blk_unregister_queue+0x53/0x180
but task is already holding lock:
ff1100011d9d00e0 (&q->q_usage_counter(queue)#3){++++}-{0:0}, at: del_gendisk+0xba/0x110
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #2 (&q->q_usage_counter(queue)#3){++++}-{0:0}:
blk_queue_enter+0x40b/0x470
blkg_conf_prep+0x7b/0x3c0
tg_set_limit+0x10a/0x3e0
cgroup_file_write+0xc6/0x420
kernfs_fop_write_iter+0x189/0x280
vfs_write+0x256/0x490
ksys_write+0x83/0x190
__x64_sys_write+0x21/0x30
x64_sys_call+0x4608/0x4630
do_syscall_64+0xdb/0x6b0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
-> #1 (&q->rq_qos_mutex){+.+.}-{4:4}:
__mutex_lock+0xd8/0xf50
mutex_lock_nested+0x2b/0x40
wbt_init+0x17e/0x280
wbt_enable_default+0xe9/0x140
blk_register_queue+0x1da/0x2e0
__add_disk+0x38c/0x5d0
add_disk_fwnode+0x89/0x250
device_add_disk+0x18/0x30
virtblk_probe+0x13a3/0x1800
virtio_dev_probe+0x389/0x610
really_probe+0x136/0x620
__driver_probe_device+0xb3/0x230
driver_probe_device+0x2f/0xe0
__driver_attach+0x158/0x250
bus_for_each_dev+0xa9/0x130
driver_attach+0x26/0x40
bus_add_driver+0x178/0x3d0
driver_register+0x7d/0x1c0
__register_virtio_driver+0x2c/0x60
virtio_blk_init+0x6f/0xe0
do_one_initcall+0x94/0x540
kernel_init_freeable+0x56a/0x7b0
kernel_init+0x2b/0x270
ret_from_fork+0x268/0x4c0
ret_from_fork_asm+0x1a/0x30
-> #0 (&q->sysfs_lock){+.+.}-{4:4}:
__lock_acquire+0x1835/0x2940
lock_acquire+0xf9/0x450
__mutex_lock+0xd8/0xf50
mutex_lock_nested+0x2b/0x40
blk_unregister_queue+0x53/0x180
__del_gendisk+0x226/0x690
del_gendisk+0xba/0x110
sd_remove+0x49/0xb0 [sd_mod]
device_remove+0x87/0xb0
device_release_driver_internal+0x11e/0x230
device_release_driver+0x1a/0x30
bus_remove_device+0x14d/0x220
device_del+0x1e1/0x5a0
__scsi_remove_device+0x1ff/0x2f0
scsi_remove_device+0x37/0x60
sdev_store_delete+0x77/0x100
dev_attr_store+0x1f/0x40
sysfs_kf_write+0x65/0x90
kernfs_fop_write_iter+0x189/0x280
vfs_write+0x256/0x490
ksys_write+0x83/0x190
__x64_sys_write+0x21/0x30
x64_sys_call+0x4608/0x4630
do_syscall_64+0xdb/0x6b0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
other info that might help us debug this:
Chain exists of:
&q->sysfs_lock --> &q->rq_qos_mutex --> &q->q_usage_counter(queue)#3
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&q->q_usage_counter(queue)#3);
lock(&q->rq_qos_mutex);
lock(&q->q_usage_counter(queue)#3);
lock(&q->sysfs_lock);
Root cause is that queue_usage_counter is grabbed with rq_qos_mutex
held in blkg_conf_prep(), while queue should be freezed before
rq_qos_mutex from other context.
The blk_queue_enter() from blkg_conf_prep() is used to protect against
policy deactivation, which is already protected with blkcg_mutex, hence
convert blk_queue_enter() to blkcg_mutex to fix this problem. Meanwhile,
consider that blkcg_mutex is held after queue is freezed from policy
deactivation, also convert blkg_alloc() to use GFP_NOIO.