CVE-2025-68178: 
Linux Kernel vulnerability analysis and mitigation

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.