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

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Date:	Sat, 11 Jul 2009 12:14:47 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Sonny Rao <sonnyrao@...ibm.com>, 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

On Sat, 11 Jul 2009, Ingo Molnar wrote:
> 
> * 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?

Acked-by-me.
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