Re: [PATCH v2] block, bfq: fix possible uaf for 'bfqq->bic'

[Jan Kara]

On Wed 14-12-22 11:04:30, 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
> 
> 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+0xe8/0x130
>  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
>  do_syscall_64+0x33/0x40
>  entry_SYSCALL_64_after_hwframe+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.
> 
> Fixes: 3bc5e683c67d ("bfq: Split shared queues on move between cgroups")
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza


> ---
> Changes in v2:
>  - Use a new solution as suggested by Jan.
> 
>  block/bfq-iosched.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index a72304c728fc..b111a7b8dca6 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -386,6 +386,12 @@ static void bfq_put_stable_ref(struct bfq_queue *bfqq);
>  
>  void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync)
>  {
> +	struct bfq_queue *old_bfqq = bic->bfqq[is_sync];
> +
> +	/* Clear bic pointer if bfqq is detached from this bic */
> +	if (old_bfqq && old_bfqq->bic == bic)
> +		old_bfqq->bic = NULL;
> +
>  	/*
>  	 * If bfqq != NULL, then a non-stable queue merge between
>  	 * bic->bfqq and bfqq is happening here. This causes troubles
> @@ -5311,7 +5317,6 @@ static void bfq_exit_icq_bfqq(struct bfq_io_cq *bic, bool is_sync)
>  		unsigned long flags;
>  
>  		spin_lock_irqsave(&bfqd->lock, flags);
> -		bfqq->bic = NULL;
>  		bfq_exit_bfqq(bfqd, bfqq);
>  		bic_set_bfqq(bic, NULL, is_sync);
>  		spin_unlock_irqrestore(&bfqd->lock, flags);
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR