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

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Date:	Sun, 24 Aug 2014 19:41:57 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>
CC:	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 08/23/2014 04:43 PM, Oleg Nesterov wrote:
> The ->start_stack check in do_shmat() looks ugly and simply wrong.
>
> 1. ->start_stack is only valid right after exec(), the application
>     can switch to another stack and even unmap this area.
>
> 2. The reason for this check is not clear at all. The application
>     should know what it does. And why 4 pages? And why in fact it
>     requires 5 pages?
>
> 3. This wrongly assumes that the stack can only grown down.
>
> Personally I think we should simply kill this check, but I did not
> dare to do this. So the patch only fixes the 1st problem (mostly to
> avoid the usage of mm->start_stack) and ignores VM_GROWSUP.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Acked-by: Manfred Spraul <manfred@...orfullife.com>
> [snip]

> +		if (vma) {
> +			if (vma->vm_flags & VM_GROWSDOWN)
> +				end += PAGE_SIZE * 4; /* can't overflow */
Why is an overflow impossible?
> +			if (end > vma->vm_start)
> +				goto invalid;
> +		}
>   	}
>   
>   	addr = do_mmap_pgoff(file, addr, size, prot, flags, 0, &populate);
--
     Manfred
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