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Subject: Re: [PATCH V1] block: Fix null pointer dereference issue on struct
 io_cq
To:     Pradeep P V K <quic_pragalla@quicinc.com>, axboe@kernel.dk
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20230517084434.18932-1-quic_pragalla@quicinc.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <07b8b870-a464-25a9-c0a6-c123fad05ff5@huaweicloud.com>
Date:   Wed, 17 May 2023 16:58:11 +0800
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Precedence: bulk

Hi,

?? 2023/05/17 16:44, Pradeep P V K д??:
> There is a potential race between ioc_clear_fn() and
> exit_io_context() as shown below, due to which below
> crash is observed. It can also result into use-after-free
> issue.
> 
> context#1:                           context#2:
> ioc_release_fn()                     do_exit();
> ->spin_lock(&ioc->lock);             ->exit_io_context();
> ->ioc_destroy_icq(icq);              ->ioc_exit_icqs();
>   ->list_del_init(&icq->q_node);       ->spin_lock_irq(&ioc->lock);
>   ->call_rcu(&icq->__rcu_head,
>       icq_free_icq_rcu);
> ->spin_unlock(&ioc->lock);
>                                        ->ioc_exit_icq(); gets the same icq
I don't understand how is this possible, the list is protected by
'ioc->lock', both hlist_del_init and hlist_for_each_entry are called
inside the lock.

Thanks,
Kuai
> 				       ->bfq_exit_icq();
>                                    This results into below crash as bic
> 				  is NULL as it is derived from icq.
> 				  There is a chance that icq could be
> 				  free'd as well.
> 
> [33.245722][ T8666] Unable to handle kernel NULL pointer dereference
> at virtual address 0000000000000018.
> ...
> Call trace:
> [33.325782][ T8666]  bfq_exit_icq+0x28/0xa8
> [33.325785][ T8666]  exit_io_context+0xcc/0x100
> [33.325786][ T8666]  do_exit+0x764/0xa58
> [33.325791][ T8666]  do_group_exit+0x0/0xa0
> [33.325793][ T8666]  invoke_syscall+0x48/0x114
> [33.325802][ T8666]  el0_svc_common+0xcc/0x118
> [33.325805][ T8666]  do_el0_svc+0x34/0xd0
> [33.325807][ T8666]  el0_svc+0x38/0xd0
> [33.325812][ T8666]  el0t_64_sync_handler+0x8c/0xfc
> [33.325813][ T8666]  el0t_64_sync+0x1a0/0x1a4
> 
> Fix this by checking with ICQ_DESTROYED flags in ioc_exit_icqs().
> Also, ensure ioc_exit_icq() is accessing icq within rcu_read_lock/unlock
> so that icq doesn't get free'd up while it is still using it.
> 
> Signed-off-by: Pradeep P V K <quic_pragalla@quicinc.com>
> ---
>   block/blk-ioc.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-ioc.c b/block/blk-ioc.c
> index 63fc02042408..1aa34fd46ac8 100644
> --- a/block/blk-ioc.c
> +++ b/block/blk-ioc.c
> @@ -60,10 +60,14 @@ static void ioc_exit_icqs(struct io_context *ioc)
>   {
>   	struct io_cq *icq;
>   
> +	rcu_read_lock();
>   	spin_lock_irq(&ioc->lock);
> -	hlist_for_each_entry(icq, &ioc->icq_list, ioc_node)
> -		ioc_exit_icq(icq);
> +	hlist_for_each_entry(icq, &ioc->icq_list, ioc_node) {
> +		if (!(icq->flags & ICQ_DESTROYED))
> +			ioc_exit_icq(icq);
> +	}
>   	spin_unlock_irq(&ioc->lock);
> +	rcu_read_unlock();
>   }
>   
>   /*
>