CVE-2022-50488

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

block, bfq: fix possible uaf for 'bfqq->bic'

Our test report a uaf for 'bfqq->bic' in 5.10:

================================================================== BUG: KASAN: use-after-free in bfqselectqueue+0x378/0xa30

CPU: 6 PID: 2318352 Comm: fsstress Kdump: loaded Not tainted 5.10.0-60.18.0.50.h602.kasan.eulerosv2r11.x8664 #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58-20220320160524-szxrtosci10000 04/01/2014 Call Trace: bfqselectqueue+0x378/0xa30 bfqdispatchrequest+0xe8/0x130 blkmqdodispatchsched+0x62/0xb0 __blkmqscheddispatchrequests+0x215/0x2a0 blkmqscheddispatchrequests+0x8f/0xd0 __blkmqrunhwqueue+0x98/0x180 __blkmqdelay_runhwqueue+0x22b/0x240 blkmqrunhwqueue+0xe3/0x190 blkmqschedinsertrequests+0x107/0x200 blkmqflushpluglist+0x26e/0x3c0 blkfinishplug+0x63/0x90 __iomapdiorw+0x7b5/0x910 iomapdiorw+0x36/0x80 ext4dioreaditer+0x146/0x190 [ext4] ext4filereaditer+0x1e2/0x230 [ext4] newsyncread+0x29f/0x400 vfsread+0x24e/0x2d0 ksysread+0xd5/0x1b0 dosyscall64+0x33/0x40 entrySYSCALL64afterhwframe+0x61/0xc6

Commit 3bc5e683c67d ("bfq: Split shared queues on move between cgroups") changes that move process to a new cgroup will allocate a new bfqq to use, however, the old bfqq and new bfqq can point to the same bic:

1) Initial state, two process with io in the same cgroup.

Process 1 Process 2 (BIC1) (BIC2) | Λ | Λ | | | | V | V | bfqq1 bfqq2

2) bfqq1 is merged to bfqq2.

Process 1 Process 2 (BIC1) (BIC2) | | -------------\| V bfqq1 bfqq2(coop)

3) Process 1 exit, then issue new io(denoce IOA) from Process 2.

(BIC2) | Λ | | V | bfqq2(coop)

4) Before IOA is completed, move Process 2 to another cgroup and issue io.

Process 2 (BIC2) Λ |--------------\ | V bfqq2 bfqq3

Now that BIC2 points to bfqq3, while bfqq2 and bfqq3 both point to BIC2. If all the requests are completed, and Process 2 exit, BIC2 will be freed while there is no guarantee that bfqq2 will be freed before BIC2.

Fix the problem by clearing bfqq->bic while bfqq is detached from bic.