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

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Message-ID: <3b98220d-5c2e-b769-1f55-ceb9565379a2@zytor.com>
Date:   Mon, 18 Feb 2019 18:46:06 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Julien Thierry <julien.thierry@....com>,
        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 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.

	-hpa