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Subject: Re: [patch V2 21/28] x86/speculation: Prepare for conditional IBPB in switch_mm()
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1811252316560.1684@nanos.tec.linutronix.de>
Date:   Sun, 25 Nov 2018 15:04:33 -0800
Cc:     Andi Kleen <ak@linux.intel.com>,
        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>
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Message-Id: <92833780-41BE-446E-A676-925BA1EC93D9@amacapital.net>
References: <20181125183328.318175777@linutronix.de> <20181125185005.466447057@linutronix.de> <20181125205330.GO13936@tassilo.jf.intel.com> <alpine.DEB.2.21.1811252316560.1684@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 25, 2018, at 2:20 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
> On Sun, 25 Nov 2018, Andi Kleen wrote:
>=20
>>> The current check whether two tasks belong to the same context is using t=
he
>>> tasks context id. While correct, it's simpler to use the mm pointer beca=
use
>>> it allows to mangle the TIF_SPEC_IB bit into it. The context id based
>>> mechanism requires extra storage, which creates worse code.
>>=20
>> [We tried similar in some really early versions, but it was replaced
>> with the context id later.]
>>=20
>> 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.
>>=20
>> 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.
>=20
> Sorry, but that's really a purely academic exercise.=20
>=20
>=20

I would guess that it=E2=80=99s actually very easy to force mm_struct* reuse=
.  Don=E2=80=99t the various allocators try to allocate hot memory?  There=E2=
=80=99s 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 ext=
ra bit, how about:

2*ctx_id vs 2*ctx_id+1

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

=E2=80=94Andy=