linux-kernel - Re: mpol_to

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Message-ID: <20121008151552.GA10881@redhat.com>
Date:	Mon, 8 Oct 2012 11:15:52 -0400
From:	Dave Jones <davej@...hat.com>
To:	Linux Kernel <linux-kernel@...r.kernel.org>,
	bhutchings@...arflare.com, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: mpol_to_str revisited.

On Mon, Oct 08, 2012 at 11:09:49AM -0400, Dave Jones wrote:
 > Last month I sent in 80de7c3138ee9fd86a98696fd2cf7ad89b995d0a to remove
 > a user triggerable BUG in mempolicy.
 > 
 > Ben Hutchings pointed out to me that my change introduced a potential leak
 > of stack contents to userspace, because none of the callers check the return value.
 > 
 > This patch adds the missing return checking, and also clears the buffer beforehand.
 > 
 > Reported-by: Ben Hutchings <bhutchings@...arflare.com>
 > Cc: stable@...nel.org
 > Signed-off-by: Dave Jones <davej@...hat.com>
 > 
 > --- 
 > unanswered question: why are the buffer sizes here different ? which is correct?

A further unanswered question is how the state got so screwed up that we hit that
default case at all.  Looking at the original report: https://lkml.org/lkml/2012/9/6/356
What's in RAX looks suspiciously like left-over slab poison.

If pol->mode was poisoned, that smells like we have a race where policy is getting freed
while another process is reading it.

Am I missing something, or is there no locking around that at all ?

	Dave

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