linux-kernel - Re: [PATCH v2] avoid null pointer access in vm

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Message-Id: <20110819155238.b11d19fb.akpm@linux-foundation.org>
Date:	Fri, 19 Aug 2011 15:52:38 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mitsuo Hayasaka <mitsuo.hayasaka.hu@...achi.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	yrl.pp-manager.tt@...achi.com, Namhyung Kim <namhyung@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH v2] avoid null pointer access in vm_struct

On Fri, 19 Aug 2011 19:51:33 +0900
Mitsuo Hayasaka <mitsuo.hayasaka.hu@...achi.com> wrote:

> The /proc/vmallocinfo shows information about vmalloc allocations in vmlist
> that is a linklist of vm_struct. It, however, may access pages field of
> vm_struct where a page was not allocated, which results in a null pointer
> access and leads to a kernel panic.
> 
> Why this happen:
> In __vmalloc_area_node(), the nr_pages field of vm_struct are set to the
> expected number of pages to be allocated, before the actual pages
> allocations. At the same time, when the /proc/vmallocinfo is read, it
> accesses the pages field of vm_struct according to the nr_pages field at
> show_numa_info(). Thus, a null pointer access happens.
> 
> Patch:
> This patch sets nr_pages field of vm_struct AFTER the pages allocations
> finished in __vmalloc_area_node(). So, it can avoid accessing the pages
> field with unallocated page when show_numa_info() is called.

I think this is still just a workaround to fix up the real bug, and
that the real bug is that the vm_struct is installed into the vmlist
*before* it is fully initialised.  It's just wrong to insert an object
into a globally-visible list and to then start populating it!  If we
were instead to fully initialise the vm_struct and *then* insert it
into vmlist, the bug is fixed.

Also I'd agree with Paul's concern regarding cross-CPU memory ordering.

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