linux-kernel - Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch

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Date:   Tue, 19 Feb 2019 09:07:00 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-arm-kernel@...ts.infradead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>, valentin.schneider@....com,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/x86: Save [ER]FLAGS on context switch



On 19/02/2019 02:46, H. Peter Anvin wrote:
> On 2/18/19 6:20 PM, Andy Lutomirski wrote:
>>
>>
>>> On Feb 18, 2019, at 4:24 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>>>
>>>> On Mon, Feb 18, 2019 at 2:31 PM H. Peter Anvin <hpa@...or.com> wrote:
>>>>
>>>> The question is what "fix it" means. I'm really concerned about AC escapes,
>>>> and everyone else should be, too.
>>>
>>> I do think that it might be the right thing to do to add some kind of
>>> WARN_ON_ONCE() for AC being set in various can-reschedule situations.
>>>
>>> We'd just have to abstract it sanely. I'm sure arm64 has the exact
>>> same issue with PAN - maybe it saves properly, but the same "we
>>> wouldn't want to go through the scheduler with PAN clear".
>>>
>>> On x86, we might as well check DF at the same time as AC.
>>>
>>
>> hpa is right, though — calling into tracing code with AC set is not really so good.  And calling schedule() (via preempt_enable() or whatever) is also bad because it runs all the scheduler code with AC on.  Admittedly, the scheduler is not *that* interesting of an attack surface.
>>
> 
> Not just that, but the other question is just how much code we are running
> with AC open. It really should only be done in some very small regions.

Yes, but we don't really have a way to enforce that, as far as I'm aware.

The user_access_begin/end() is generic API, meaning any arch is free to
implement it. If they don't have the same hardware behaviour as
x86/arm64, it might be that their interrupt/exception entry code will
run with user_access open until they reach the entry code that closes it
(and entry code could potentially be a more interesting attack surface
than the scheduler). This could be the case of software emulated PAN on
arm/arm64 (although currently arm, non-64bit, doesn't have
user_access_begin/end() at the time).

So the whole "very small region" restriction sounds a bit
loose/arbitrary to me...

Thanks,

-- 
Julien Thierry