linux-kernel - Re: [PATCH 1/3] futex: do not pagefault_disable in futex_atomic_cmpxchg

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Date:	Mon, 14 Mar 2011 13:07:10 -0700
From:	Darren Hart <darren.hart@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Darren Hart <darren@...art.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Matt Turner <mattst88@...il.com>,
	Russell King <linux@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Tony Luck <tony.luck@...el.com>,
	Michal Simek <monstr@...str.eu>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	"David S. Miller" <davem@...emloft.net>,
	Chris Metcalf <cmetcalf@...era.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] futex: do not pagefault_disable in futex_atomic_cmpxchg_inatomic()

On 03/14/2011 06:56 AM, Thomas Gleixner wrote:
> On Mon, 14 Mar 2011, Thomas Gleixner wrote:
>> On Sun, 13 Mar 2011, Linus Torvalds wrote:
>>> On Thu, Mar 10, 2011 at 6:47 PM, Michel Lespinasse<walken@...gle.com>  wrote:
>> That's my fault.
>>
>> I really checked the call sites of futex_atomic_cmpxchg_inatomic() and
>> totally failed to see the one in handle_futex_death() which does not
>> use the helper function cmpxchg_futex_value_locked(). That helper
>> function is safe and does the right thing:
>>
>> 	pagefault_disable();
>> 	curval = futex_atomic_cmpxchg_inatomic(uaddr, uval, newval);
>> 	pagefault_enable();
>>
>> So, that means we have all call sites covered except one, which needs
>> to be fixed _AND_ also pushed into stable as all arch implementations
>> except ARM rely on the caller doing the pagefault_disable().
>
> After applying some coffee to my brain, I noticed that the ability to
> fault in handle_futex_death() is desired. The get_user() before that
> call covers the case where the futex is paged out, but it does not
> handle the case where the futex is in a non writeable mapping. That
> lacks a big fat comment at least.
>
> So the removal of the pagefault_disable() in ARM is correct, just the
> changelog and the comment there sucks. Sorry for not catching it.
>
> Thinking more about it. Adding a comment is to handle_futex_death() is
> good, but changing the code to make it entirely clear what is going on
> is even better.
>
> -------->
> Subject: futex: Deobfuscate handle_futex_death()
> From: Thomas Gleixner<tglx@...utronix.de>
> Date: Mon, 14 Mar 2011 10:34:35 +0100
>
> handle_futex_death() uses futex_atomic_cmpxchg_inatomic() without
> disabling page faults. That's ok, but totally non obvious.
>
> We don't hold locks so we actually can and want to fault here, because
> the get_user() before futex_atomic_cmpxchg_inatomic() does not
> guarantee a R/W mapping.
>
> We could just add a big fat comment to explain this, but actually
> changing the code so that the functionality is entirely clear is
> better.
>
> Use the helper function which disables page faults around the
> futex_atomic_cmpxchg_inatomic() and handle a fault with a call to
> fault_in_user_writeable() as all other places in the futex code do as
> well.
>
> Signed-off-by: Thomas Gleixner<tglx@...utronix.de>

Acked-by: Darren Hart <dvhart@...ux.intel.com>

> ---
>   kernel/futex.c |   17 ++++++++++++++---
>   1 file changed, 14 insertions(+), 3 deletions(-)
>
> Index: linux-2.6-tip/kernel/futex.c
> ===================================================================
> --- linux-2.6-tip.orig/kernel/futex.c
> +++ linux-2.6-tip/kernel/futex.c
> @@ -2458,9 +2458,20 @@ retry:
>   		 * userspace.
>   		 */
>   		mval = (uval&  FUTEX_WAITERS) | FUTEX_OWNER_DIED;
> -		if (futex_atomic_cmpxchg_inatomic(&nval, uaddr, uval, mval))
> -			return -1;
> -
> +		/*
> +		 * We are not holding a lock here, but we want to have
> +		 * the pagefault_disable/enable() protection because
> +		 * we want to handle the fault gracefully. If the
> +		 * access fails we try to fault in the futex with R/W
> +		 * verification via get_user_pages. get_user() above
> +		 * does not guarantee R/W access. If that fails we
> +		 * give up and leave the futex locked.
> +		 */
> +		if (cmpxchg_futex_value_locked(&nval, uaddr, uval, mval)) {
> +			if (fault_in_user_writeable(uaddr))
> +				return -1;
> +			goto retry;
> +		}
>   		if (nval != uval)
>   			goto retry;
>
> --
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-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
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