linux-kernel - Re: general protection fault in put

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Date:   Thu, 3 Jan 2019 14:18:57 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot+1145ec2e23165570c3ac@...kaller.appspotmail.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        ktsanaktsidis@...desk.com, LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Matthew Wilcox <willy@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: general protection fault in put_pid

Hi Manfred,

On Sun, Dec 23, 2018 at 4:26 AM Manfred Spraul <manfred@...orfullife.com> wrote:
>
> Hello Dmitry,
>
> On 12/23/18 10:57 AM, Dmitry Vyukov wrote:
> >
> > I can reproduce this infinite memory consumption with the C program:
> > https://gist.githubusercontent.com/dvyukov/03ec54b3429ade16fa07bf8b2379aff3/raw/ae4f654e279810de2505e8fa41b73dc1d77778e6/gistfile1.txt
> >
> > But this is working as intended, right? It just creates infinite
> > number of large semaphore sets, which reasonably consumes infinite
> > amount of memory.
> > Except that it also violates the memcg bound and a process can have
> > effectively unlimited amount of such "drum memory" in semaphores.
>
> Yes, this is as intended:
>
> If you call semget(), then you can use memory, up to the limits in
> /proc/sys/kernel/sem.
>
> Memcg is not taken into account, an admin must set /proc/sys/kernel/sem.
>
> The default are "infinite amount of memory allowed", as this is the most
> sane default: We had a logic that tried to autotune (i.e.: a new
> namespace "inherits" a fraction of the parent namespaces memory limits),
> but this we more or less always wrong.
>
>

What's the disadvantage of setting the limits in /proc/sys/kernel/sem
high and let the task's memcg limits the number of semaphore a process
can create? Please note that the memory underlying shmget and msgget
is already accounted to memcg.

thanks,
Shakeel