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

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Message-Id: <92833780-41BE-446E-A676-925BA1EC93D9@amacapital.net>
Date:   Sun, 25 Nov 2018 15:04:33 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        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 Nov 25, 2018, at 2:20 PM, Thomas Gleixner <tglx@...utronix.de> 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. 
> 
> 

I would guess that it’s actually very easy to force mm_struct* reuse.  Don’t the various allocators try to allocate hot memory?  There’s nothing hotter than a just-freed allocation of the same size.

Can someone explain the actual problem with ctx_id?  If you just need an extra bit, how about:

2*ctx_id vs 2*ctx_id+1

Or any of the many variants of approximately the same thing?

—Andy