linux-kernel - Re: [PATCH v2 08/12] cachefiles: never get a new anonymous fd if ondemand

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Date: Mon, 20 May 2024 19:24:06 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Baokun Li <libaokun@...weicloud.com>,
 Jingbo Xu <jefflexu@...ux.alibaba.com>, netfs@...ts.linux.dev,
 dhowells@...hat.com, jlayton@...nel.org
Cc: zhujia.zj@...edance.com, linux-erofs@...ts.ozlabs.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 yangerkun@...wei.com, houtao1@...wei.com, yukuai3@...wei.com,
 wozizhi@...wei.com, Baokun Li <libaokun1@...wei.com>
Subject: Re: [PATCH v2 08/12] cachefiles: never get a new anonymous fd if
 ondemand_id is valid



On 2024/5/20 19:14, Baokun Li wrote:
> On 2024/5/20 17:24, Jingbo Xu wrote:
>>
>> On 5/20/24 5:07 PM, Baokun Li wrote:
>>> On 2024/5/20 16:43, Jingbo Xu wrote:
>>>> On 5/15/24 4:45 PM, libaokun@...weicloud.com wrote:
>>>>> From: Baokun Li <libaokun1@...wei.com>
>>>>>
> SNIP
>>>>>
>>>>> To avoid this, allocate a new anonymous fd only if no anonymous fd has
>>>>> been allocated (ondemand_id == 0) or if the previously allocated
>>>>> anonymous
>>>>> fd has been closed (ondemand_id == -1). Moreover, returns an error if
>>>>> ondemand_id is valid, letting the daemon know that the current userland
>>>>> restore logic is abnormal and needs to be checked.
>>>>>
>>>>> Fixes: c8383054506c ("cachefiles: notify the user daemon when looking
>>>>> up cookie")
>>>>> Signed-off-by: Baokun Li <libaokun1@...wei.com>
>>>> The LOCs of this fix is quite under control.  But still it seems that
>>>> the worst consequence is that the (potential) malicious daemon gets
>>>> hung.  No more effect to the system or other processes.  Or does a
>>>> non-malicious daemon have any chance having the same issue?
>>> If we enable hung_task_panic, it may cause panic to crash the server.
>> Then this issue has nothing to do with this patch?  As long as a
>> malicious daemon doesn't close the anonymous fd after umounting, then I
>> guess a following attempt of mounting cookie with the same name will
>> also wait and hung there?
>>
> Yes, a daemon that only reads requests but doesn't process them will
> cause hung，but the daemon will obey the basic constraints when we
> test it.

If we'd really like to enhanace this ("hung_task_panic"), I think
you'd better to switch wait_for_completion() to
wait_for_completion_killable() at least IMHO anyway.

Thanks,
Gao Xiang