linux-kernel - Re: [RFC]: Possible race condition in kernel futex code

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Message-Id: <4387EFE3-55AB-433B-83E1-2AFC413C7B9A@online.nl>
Date:	Fri, 9 Oct 2015 11:49:45 +0200
From:	Hans Zuidam <h.zuidam@...ine.nl>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Jaccon Bastiaansen <jaccon.bastiaansen@...il.com>, x86@...nel.org,
	mingo@...hat.com, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable@...r.kernel.org
Subject: Re: [RFC]: Possible race condition in kernel futex code

Hi Thomas,

On 9 okt. 2015, at 11:06, Thomas Gleixner <tglx@...utronix.de> wrote:
On Mon, 5 Oct 2015, Jaccon Bastiaansen wrote:
>> We did some tests with different compilers, kernel versions and kernel
>> configs, with the following results:

> You cannot use an explicit 32bit read. We need an access which handles the fault gracefully.

The reason for the explicit read suggestion is to avoid the _builtin_constant_p() in __copy_from_user_nocheck().  The GCC manual says that there may be situations where it returns 0 even though the argument is a constant.  Although none of the compiler/kernel combinations we have tried showed this happening, we think it is probably better to be safe than sorry.

With kind regards,
Hans Zuidam

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