linux-kernel - Re: [PATCH] memcg: avoid "free" overflow in memcg_hierarchical_free

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Message-ID: <20101109091006.GR23393@cmpxchg.org>
Date:	Tue, 9 Nov 2010 10:10:06 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Minchan Kim <minchan.kim@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: avoid "free" overflow in
 memcg_hierarchical_free_pages()

On Tue, Nov 09, 2010 at 12:54:13AM -0800, Greg Thelen wrote:
> memcg limit and usage values are stored in res_counter, as 64-bit
> numbers, even on 32-bit machines.  The "free" variable in
> memcg_hierarchical_free_pages() stores the difference between two
> 64-bit numbers (limit - current_usage), and thus should be stored
> in a 64-bit local rather than a machine defined unsigned long.

It is converted to pages before the assignment, but even that might
overflow on 32-bit if the difference is sufficiently large (> 1<<44).

> Reported-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Signed-off-by: Greg Thelen <gthelen@...gle.com>

Reviewed-by: Johannes Weiner <hannes@...xchg.org>
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