On Sat 10-12-22 18:25:37, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Our test report a uaf for 'bfqq->bic' in 5.10:
>
> ==================================================================
> BUG: KASAN: use-after-free in bfq_select_queue+0x378/0xa30
> Read of size 8 at addr ffff88810efb42d8 by task fsstress/2318352
>
> CPU: 6 PID: 2318352 Comm: fsstress Kdump: loaded Not tainted 5.10.0-60.18.0.50.h602.kasan.eulerosv2r11.x86_64 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58-20220320_160524-szxrtosci10000 04/01/2014
> Call Trace:
...
> bfq_select_queue+0x378/0xa30
> __bfq_dispatch_request+0x1c4/0x220
> bfq_dispatch_request+0xe8/0x130
> __blk_mq_do_dispatch_sched+0x3f4/0x560
> blk_mq_do_dispatch_sched+0x62/0xb0
> __blk_mq_sched_dispatch_requests+0x215/0x2a0
> blk_mq_sched_dispatch_requests+0x8f/0xd0
> __blk_mq_run_hw_queue+0x98/0x180
> __blk_mq_delay_run_hw_queue+0x22b/0x240
> blk_mq_run_hw_queue+0xe3/0x190
> blk_mq_sched_insert_requests+0x107/0x200
> blk_mq_flush_plug_list+0x26e/0x3c0
> blk_finish_plug+0x63/0x90
> __iomap_dio_rw+0x7b5/0x910
> iomap_dio_rw+0x36/0x80
> ext4_dio_read_iter+0x146/0x190 [ext4]
> ext4_file_read_iter+0x1e2/0x230 [ext4]
> new_sync_read+0x29f/0x400
> vfs_read+0x24e/0x2d0
> ksys_read+0xd5/0x1b0
Perhaps we can trim this UAF report a bit to what I've left above? That
should be enough to give idea about the problem.
> 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（cg1)
> (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 if process references is decreased
> to zero, since that they are not related anymore.
>
> Fixes: 3bc5e683c67d ("bfq: Split shared queues on move between cgroups")
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
Thanks for the analysis and the patch! I agree this is a problem.
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index a72304c728fc..6eada99d1b34 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -3036,6 +3036,14 @@ void bfq_release_process_ref(struct bfq_data *bfqd, struct bfq_queue *bfqq)
>
> bfq_reassign_last_bfqq(bfqq, NULL);
>
> + /*
> + * __bfq_bic_change_cgroup() just reset bic->bfqq so that a new bfqq
> + * will be created to handle new io, while old bfqq will stay around
> + * until all the requests are completed. It's unsafe to keep bfqq->bic
> + * since they are not related anymore.
> + */
> + if (bfqq_process_refs(bfqq) == 1)
> + bfqq->bic = NULL;
> bfq_put_queue(bfqq);
Rather than changing bfq_release_process_ref() I think it would be more
logical to change bic_set_bfqq() like:
struct bfq_queue *old_bfqq = bic->bfqq[is_sync];
/* Clear bic pointer if we are detaching bfqq from its bic */
if (old_bfqq && old_bfqq->bic == bic)
old_bfqq->bic = NULL;
And then we can also remove several explicit bfqq->bic = NULL statements
from bfq code.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
