CVE-2024-56672

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

blk-cgroup: Fix UAF in blkcgunpinonline()

blkcgunpinonline() walks up the blkcg hierarchy putting the online pin. To walk up, it uses blkcgparent(blkcg) but it was calling that after blkcgdestroy_blkgs(blkcg) which could free the blkcg, leading to the following UAF:

================================================================== BUG: KASAN: slab-use-after-free in blkcgunpinonline+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: cgwbrelease cgwbreleaseworkfn Call Trace: <TASK> dumpstacklvl+0x27/0x80 printreport+0x151/0x710 kasanreport+0xc0/0x100 blkcgunpinonline+0x15a/0x270 cgwbreleaseworkfn+0x194/0x480 processscheduledworks+0x71b/0xe20 workerthread+0x82a/0xbd0 kthread+0x242/0x2c0 retfromfork+0x33/0x70 retfromforkasm+0x1a/0x30 </TASK> ... Freed by task 1944: kasansavetrack+0x2b/0x70 kasansavefreeinfo+0x3c/0x50 _kasanslabfree+0x33/0x50 kfree+0x10c/0x330 cssfreerworkfn+0xe6/0xb30 processscheduledworks+0x71b/0xe20 workerthread+0x82a/0xbd0 kthread+0x242/0x2c0 retfromfork+0x33/0x70 retfromforkasm+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 blkcgunpinonline().

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