linux-kernel - Re: [PATCH] futexs: fix infinite loop in get_futex

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Message-ID: <20090711081924.GA10437@elte.hu>
Date:	Sat, 11 Jul 2009 10:19:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Sonny Rao <sonnyrao@...ibm.com>
Cc:	tglx@...uxtronix.de, mingo@...hat.com,
	linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org,
	stable@...nel.org, anton@...ba.org, rajamony@...ibm.com,
	speight@...ibm.com, mstephen@...ibm.com, grimm@...ibm.com,
	mikey@...abs.au.ibm.com
Subject: Re: [PATCH] futexs: fix infinite loop in get_futex_key on huge page


* Sonny Rao <sonnyrao@...ibm.com> wrote:

> get_futex_key() can infinitely loop if it is called on a virtual 
> address that is within a huge page but not aligned to the 
> beginning of that page.  The call to get_user_pages_fast will 
> return the struct page for a sub-page within the huge page and the 
> check for page->mapping will always fail.
> 
> The fix is to call compound_head on the page before checking that 
> it's mapped.
> 
> Signed-off-by: Sonny Rao <sonnyrao@...ibm.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: stable@...nel.org
> 
> Index: linux-2.6.31-rc2/kernel/futex.c
> ===================================================================
> --- linux-2.6.31-rc2.orig/kernel/futex.c	2009-07-10 17:45:46.181084475 -0500
> +++ linux-2.6.31-rc2/kernel/futex.c	2009-07-10 17:46:47.345084062 -0500
> @@ -247,6 +247,7 @@
>  	if (err < 0)
>  		return err;
>  
> +	page = compound_head(page);
>  	lock_page(page);
>  	if (!page->mapping) {
>  		unlock_page(page);

Nice catch! Applied it to tip:core/urgent - Thomas, do you agree 
with the fix?

	Ingo
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