CVE-2024-56672

In the Linux kernel, the following vulnerability has been resolved:

blk-cgroup: Fix UAF in blkcg_unpin_online()

blkcg_unpin_online() walks up the blkcg hierarchy putting the online pin. To

walk up, it uses blkcg_parent(blkcg) but it was calling that after

blkcg_destroy_blkgs(blkcg) which could free the blkcg, leading to the

following UAF:

==================================================================

BUG: KASAN: slab-use-after-free in blkcg_unpin_online+0x15a/0x270

Read of size 8 at addr ffff8881057678c0 by task kworker/9:1/117

CPU: 9 UID: 0 PID: 117 Comm: kworker/9:1 Not tainted 6.13.0-rc1-work-00182-gb8f52214c61a-dirty #48

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS unknown 02/02/2022

Workqueue: cgwb_release cgwb_release_workfn

Call Trace:

<TASK>

dump_stack_lvl+0x27/0x80

print_report+0x151/0x710

kasan_report+0xc0/0x100

blkcg_unpin_online+0x15a/0x270

cgwb_release_workfn+0x194/0x480

process_scheduled_works+0x71b/0xe20

worker_thread+0x82a/0xbd0

kthread+0x242/0x2c0

ret_from_fork+0x33/0x70

ret_from_fork_asm+0x1a/0x30

</TASK>

...

Freed by task 1944:

kasan_save_track+0x2b/0x70

kasan_save_free_info+0x3c/0x50

__kasan_slab_free+0x33/0x50

kfree+0x10c/0x330

css_free_rwork_fn+0xe6/0xb30

process_scheduled_works+0x71b/0xe20

worker_thread+0x82a/0xbd0

kthread+0x242/0x2c0

ret_from_fork+0x33/0x70

ret_from_fork_asm+0x1a/0x30

Note that the UAF is not easy to trigger as the free path is indirected

behind a couple RCU grace periods and a work item execution. I could only

trigger it with artifical msleep() injected in blkcg_unpin_online().

Fix it by reading the parent pointer before destroying the blkcg's blkg's.