linux-kernel - Re: [patch V4 part 4 02/24] x86/int3: Avoid atomic instrumentation

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Date:   Thu, 14 May 2020 08:08:46 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <JGross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 4 02/24] x86/int3: Avoid atomic instrumentation



> On May 14, 2020, at 8:06 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> Peter Zijlstra <peterz@...radead.org> writes:
>>> On Thu, May 14, 2020 at 02:51:32PM +0200, Thomas Gleixner wrote:
>>> Peter Zijlstra <peterz@...radead.org> writes:
>>>> On Wed, May 13, 2020 at 09:57:52PM -0700, Andy Lutomirski wrote:
>>>>> On Tue, May 5, 2020 at 7:15 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>>>>> 
>>>>>> From: Peter Zijlstra <peterz@...radead.org>
>>>>>> 
>>>>>> Use arch_atomic_*() and READ_ONCE_NOCHECK() to ensure nothing untoward
>>>>>> creeps in and ruins things.
>>>>>> 
>>>>>> That is; this is the INT3 text poke handler, strictly limit the code
>>>>>> that runs in it, lest it inadvertenly hits yet another INT3.
>>>>> 
>>>>> 
>>>>> Acked-by: Andy Lutomirski <luto@...nel.org>
>>>>> 
>>>>> Does objtool catch this error?
>>>> 
>>>> It does not. I'll put it on the (endless) todo list..
>>> 
>>> Well, at least it detects when that code calls out into something which
>>> is not in the non-instrumentable section.
>> 
>> True, but the more specific problem is that noinstr code can use
>> jump_label/static_call just fine.
>> 
>> So a more specific test is validating none of that happens in the INT3
>> handler before poke_int3_handler(). Which is what I think Andy was
>> after.
> 
> Indeed. Forgot about that one.
> 
> Hmm, alternatives and jumplabel patch locations in entry.text and
> noinstr.text can be valid at least during early boot where we know that
> we don't run those code pathes...

Alternatives should be valid regardless. Isn’t the world essentially stopped while we apply them?

> 
> Thanks,
> 
>        tglx