linux-kernel - Re: [PATCH] ipc/shm: disable SHMALL, SHMMAX

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Date:	Sat, 12 Apr 2014 09:22:24 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	gthelen@...gle.com, aswin@...com, linux-mm@...ck.org
Subject: Re: [PATCH] ipc/shm: disable SHMALL, SHMMAX

On Sat, 2014-04-12 at 08:39 -0700, Davidlohr Bueso wrote:
> On Sat, 2014-04-12 at 13:48 +0200, Manfred Spraul wrote:
> > Shared memory segment can be abused to trigger out-of-memory conditions and
> > the standard measures against out-of-memory do not work:
> > 
> > - It is not possible to use setrlimit to limit the size of shm segments.
> > 
> > - Segments can exist without association with any processes, thus
> >   the oom-killer is unable to free that memory.
> > 
> > Therefore Linux always limited the size of segments by default to 32 MB.
> > As most systems do not need a protection against malicious user space apps,
> > a default that forces most admins and distros to change it doesn't make
> > sense.
> > 
> > The patch disables both limits by setting the limits to ULONG_MAX.
> > 
> > Admins who need a protection against out-of-memory conditions should
> > reduce the limits again and/or enable shm_rmid_forced.
> > 
> > Davidlohr: What do you think?
> > 
> > I prefer this approach: No need to update the man pages, smaller change
> > of the code, smaller risk of user space incompatibilities.
> 
> As I've mentioned before, both approaches are correct.
> 
> I still much prefer using 0 instead of ULONG_MAX, it's far easier to
> understand. And considering the v2 which fixes the shmget(key, 0, flg)
> usage, I _still_ don't see why it would cause legitimate user
> incompatibilities.

Also, if the user overflows the variable (indicating that he/she wants
to increase it to reflect something 'unlimited') and it ends up being 0,
then it becomes a valid value, not something totally wrong as it is
today.

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