linux-kernel - Re: [RFC][PATCH] fix move/migrate

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Message-Id: <20120229123120.127e21fd.akpm@linux-foundation.org>
Date:	Wed, 29 Feb 2012 12:31:20 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH] fix move/migrate_pages() race on task struct

On Tue, 28 Feb 2012 13:30:19 -0600 (CST)
Christoph Lameter <cl@...ux.com> wrote:

> Migration functions perform the rcu_read_unlock too early. As a result the
> task pointed to may change from under us.
> 
> The following patch extend the period of the rcu_read_lock until after the
> permissions checks are done. We also take a refcount so that the task
> reference is stable when calling security check functions and performing
> cpuset node validation (which takes a mutex).
> 
> The refcount is dropped before actual page migration occurs so there is no
> change to the refcounts held during page migration.
> 
> Also move the determination of the mm of the task struct to immediately
> before the do_migrate*() calls so that it is clear that we switch from
> handling the task during permission checks to the mm for the actual
> migration. Since the determination is only done once and we then no longer
> use the task_struct we can be sure that we operate on a specific address
> space that will not change from under us.

What was the user-visible impact of the bug?

Please always include info this in bug fix changelogs - it helps me and
others to decide which kernel version(s) the patch should be merged
into.

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