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Date:   Sun, 25 Nov 2018 19:07:08 -0800
From:   Andi Kleen <ak@linux.intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>
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()
Message-ID: <20181126030708.GP13936@tassilo.jf.intel.com>
References: <20181125183328.318175777@linutronix.de>
 <20181125185005.466447057@linutronix.de>
 <20181125205330.GO13936@tassilo.jf.intel.com>
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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