linux-kernel - Re: [PATCH] mm: fix possible cause of a page

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Message-ID: <BANLkTingV3eiHEco+36YyM4YTDHFHc9_jA@mail.gmail.com>
Date:	Mon, 18 Apr 2011 14:15:07 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Robert Święcki <robert@...ecki.net>,
	Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] mm: fix possible cause of a page_mapped BUG

On Tue, Apr 12, 2011 at 12:38 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Apr 12, 2011 at 12:02 PM, Robert Święcki <robert@...ecki.net> wrote:
>>
>> I'm testing currently with the old one, w/o any symptoms of problems
>> by now, but it's not a meaningful period of time. I can try with the
>> new one, leave it over(European)night, and let you know tomorrow.
>
> You might as well keep testing the old one, if that gives it better
> coverage. No need to disrupt anything you already have running.
>
> The more important input is "was that actually the root cause", rather
> than deciding between the ugly or clean way of fixing it.
>
> So if the first patch fixes it, then I'm pretty sure the second one
> will too - just in a cleaner manner.

Sorry for the delayed response - I have been traveling abroad in the
last two weeks and until the end of the month.

This second patch looks more attractive than the first, but is also
harder to prove correct. Hugh looked at all gup call sites and
convinced himself that the change was safe, except for the
fault_in_user_writeable() site in futex.c which he asked me to look
at. I am worried that we would have an issue there, as places like
futex_wake_op() or fixup_pi_state_owner() operate on user memory with
page faults disabled, and expect fault_in_user_writeable() to set up
the user page so that they can retry if the initial access failed.
With this proposal, fault_in_user_writeable() would become inoperative
when the  address is within the guard page; this could cause some
malicious futex operation to create an infinite loop.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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