From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vasily Averin <vvs-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
Subject: Re: [PATCH v3 15/16] memcg: enable accounting for tty-related objects
Date: Fri, 23 Apr 2021 13:29:53 +0300
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To: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>
Cc: Greg Kroah-Hartman <gregkh-hQyY1W1yCW8ekmWlsbkhG0B+6BGkLq7r@public.gmane.org>, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>, Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>, Jiri Slaby <jirislaby-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>

On 4/23/21 11:58 AM, Michal Hocko wrote:
> On Fri 23-04-21 10:53:55, Vasily Averin wrote:
>> On 4/22/21 4:59 PM, Vasily Averin wrote:
>>> On 4/22/21 2:50 PM, Greg Kroah-Hartman wrote:
>>>> On Thu, Apr 22, 2021 at 01:44:59PM +0200, Michal Hocko wrote:
>>>>> On Thu 22-04-21 13:23:21, Greg KH wrote:
>>>>>> On Thu, Apr 22, 2021 at 01:37:53PM +0300, Vasily Averin wrote:
>>>>>>> At each login the user forces the kernel to create a new terminal and
>>>>>>> allocate up to ~1Kb memory for the tty-related structures.
>>>>>>
>>>>>> Does this tiny amount of memory actually matter?
>>>>>
>>>>> The primary question is whether an untrusted user can trigger an
>>>>> unbounded amount of these allocations.
>>>>
>>>> Can they?  They are not bounded by some other resource limit?
>>>
>>> I'm not ready to provide usecase right now,
>>> but on the other hand I do not see any related limits.
>>> Let me take time out to dig this question.
>>
>> By default it's allowed to create up to 4096 ptys with 1024 reserve for initns only
>> and the settings are controlled by host admin. It's OK.
>> Though this default is not enough for hosters with thousands of containers per node.
>> Host admin can be forced to increase it up to NR_UNIX98_PTY_MAX = 1<<20.
>>
>> By default container is restricted by pty mount_opt.max = 1024, but admin inside container 
>> can change it via remount. In result one container can consume almost all allowed ptys 
>> and allocate up to 1Gb of unaccounted memory.
>>
>> It is not enough per-se to trigger OOM on host, however anyway, it allows to significantly
>> exceed the assigned memcg limit and leads to troubles on the over-committed node.
>> So I still think it makes sense to account this memory.
> 
> This is a very valuable information to have in the changelog. It is not
> my call but if all the above is correct then the accounting is worth
> IMO.

If Greg doesn't have any objections, I'll add this explanation to the next version of the patch.