linux-kernel - Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start

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Message-ID: <alpine.LSU.2.11.1408250856020.24245@eggly.anvils>
Date:	Mon, 25 Aug 2014 09:18:48 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Hugh Dickins <hughd@...gle.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Vagin <avagin@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Julien Tinnes <jln@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] ipc/shm: fix the historical/wrong mm->start_stack
 check

On Mon, 25 Aug 2014, Oleg Nesterov wrote:
> On 08/24, Hugh Dickins wrote:
> >
> > I'd say it comes earlier, from Christoph Rohland's 2.4.17-pre7's
> > "Add missing checks on shmat()", though I didn't find more than that.
> >
> > We can all understand wanting to leave a gap below the growsdown stack,
> > but of course could argue about growsup and 1 or 4 or 5 or whatever:
> 
> And it is not clear to me why the kernel should care at all,

Care about what exactly?  Leaving a gap between shm and stack?

The man page says that (unless SHM_REMAP) shmat() will fail with
EINVAL if a mapping already exists there, and I think it's fair
to regard the vm_start of a VM_GROWSDOWN somewhat elastically.

It may be that Linus's check_stack_guard_page() work in 2.6.36
changed the importance of this shmat() check, but I'd still feel
safer to leave it as is (while turning a blind eye to the
VM_GROWSUP omission).

> 
> > okay that we're all more interested in just removing that start_stack.
> 
> so perhaps v2 should simply remove it? Or do you think it would be safer
> to not do this?

It would be safer to leave it, but replace the start_stack use as you did.
And I think I'll let Linus's guard page justify your 4 (to match comment)
in place of the original's mysterious 5.

Hugh
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