linux-kernel - Re: [patch V2 21/28] x86/speculation: Prepare for conditional IBPB in switch

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Date:   Sun, 25 Nov 2018 19:07:08 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch V2 21/28] x86/speculation: Prepare for conditional IBPB
 in switch_mm()

On Sun, Nov 25, 2018 at 11:20:50PM +0100, Thomas Gleixner wrote:
> 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. 

Ok fair enough. I guess it's acceptable if you add a comment explaining it.

-Andi