linux-kernel - Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

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Date:   Thu, 29 Nov 2018 09:52:56 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>, julia@...com, jeyu@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64


> On Nov 29, 2018, at 9:45 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
>> On Thu, Nov 29, 2018 at 09:41:33AM -0800, Andy Lutomirski wrote:
>> 
>>> On Nov 29, 2018, at 9:21 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>> 
>>> On Thu, 29 Nov 2018 12:20:00 -0500
>>> Steven Rostedt <rostedt@...dmis.org> wrote:
>>> 
>>> 
>>>> r8 = return address
>>>> r9 = function to call
>>>> 
>>> 
>>> Bad example, r8 and r9 are args, but r10 and r11 are available.
>>> 
>>> -- Steve
>>> 
>>>>   push r8
>>>>   jmp *r9
>>>> 
>>>> Then have the regs->ip point to that trampoline.
>> 
>> Cute. That’ll need ORC annotations and some kind of retpoline to replace the indirect jump, though.
> 
> I'm going with this idea, but the BP is so rare that I really don't see
> why a retpoline would be needed.
> 

Without the retpoline in place, you are vulnerable to security researchers causing you a personal denial of service by finding a way to cause the BP to get hit, mistraining the branch predictor, and writing a paper about it :)