linux-kernel - Re: [RFC PATCH 2/2] x86/ibpb: Prevent missed IBPB flush

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Date:   Thu, 25 Jan 2018 08:56:57 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     David Woodhouse <dwmw2@...radead.org>, linux-kernel@...r.kernel.org
Cc:     KarimAllah Ahmed <karahmed@...zon.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, rkrcmar@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org
Subject: Re: [RFC PATCH 2/2] x86/ibpb: Prevent missed IBPB flush

On 01/25/2018 12:20 AM, David Woodhouse wrote:
> On Wed, 2018-01-24 at 16:36 -0800, Tim Chen wrote:
>> It is possible that the last uesr mm that we recorded for a cpu was
>> released, and a new mm with identical address was allocated when we
>> check it again.  We could skip IBPB flush here for the process with
>> the new mm.
>>
>> It is a difficult to exploit case as we have to exit() a process on a
>> cpu, free the mm, and fork() the victim to use the mm pointer on that
>> cpu. The exploiter needs the old mm to get recycled to the
>> newly forked process and no other processes run on the target cpu.
> 
> That's what it takes to have the victim process leak information into
> the cache. In order to *harvest* that information, the attacker must
> then get run on the same CPU again? And since her first process had to
> exits, as described above, she needs a new process for that?
> 
> I confess, with all the other wildly theoretical loopholes that exist,
> I wasn't losing much sleep over this one.
> 
>> Nevertheless, the patch below is one way to close this hole by
>> adding a ref count to prevent the last user mm from being released.
>> It does add ref counting overhead, and extra memory cost of keeping an mm
>> (though not the VMAs and most of page tables) around longer than we will
>> otherwise need to. Any better solutions are welcomed.
> 
> This has no upper bound on the amount of time the user mm gets held,
> right? If a given CPU remains idle for ever (and what happens if it's
> taken offline?) we'll never do that mmdrop() ?
> 

The downside with this approach is we do hold on to the mm longer
than we needs to.

Yes, the offline path does need to be fixed up.

Tim