Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Sun, 25 Nov 2018 23:20:50 +0100 (CET)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andi Kleen <ak@linux.intel.com>
cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jiri Kosina <jkosina@suse.cz>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Casey Schaufler <casey.schaufler@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Jon Masters <jcm@redhat.com>,
        Waiman Long <longman9394@gmail.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Dave Stewart <david.c.stewart@intel.com>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [patch V2 21/28] x86/speculation: Prepare for conditional IBPB
 in switch_mm()
In-Reply-To: <20181125205330.GO13936@tassilo.jf.intel.com>
Message-ID: <alpine.DEB.2.21.1811252316560.1684@nanos.tec.linutronix.de>
References: <20181125183328.318175777@linutronix.de> <20181125185005.466447057@linutronix.de> <20181125205330.GO13936@tassilo.jf.intel.com>
User-Agent: Alpine 2.21 (DEB 202 2017-01-01)
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sun, 25 Nov 2018, Andi Kleen wrote:

> > The current check whether two tasks belong to the same context is using the
> > tasks context id. While correct, it's simpler to use the mm pointer because
> > it allows to mangle the TIF_SPEC_IB bit into it. The context id based
> > mechanism requires extra storage, which creates worse code.
> 
> [We tried similar in some really early versions, but it was replaced
> with the context id later.]
> 
> One issue with using the pointer is that the pointer can be reused
> when the original mm_struct is freed, and then gets reallocated
> immediately to an attacker. Then the attacker may avoid the IBPB.
> 
> Given it's probably hard to generate any reasonable leak bandwidth with
> such a complex scenario, but it still seemed better to close the hole.

Sorry, but that's really a purely academic exercise. 

Thanks,

	tglx