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

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Message-Id: <ECF632D9-581B-40B7-9F0A-6B5607A4D0E3@amacapital.net>
Date:   Thu, 29 Nov 2018 05:37:39 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Steven Rostedt <rostedt@...dmis.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 1:42 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Nov 28, 2018 at 10:05:54PM -0800, Andy Lutomirski wrote:
> 
>>>> +static void static_call_bp_handler(struct pt_regs *regs, void *_data)
>>>> +{
>>>> +    struct static_call_bp_data *data = _data;
>>>> +
>>>> +    /*
>>>> +     * For inline static calls, push the return address on the stack so the
>>>> +     * "called" function will return to the location immediately after the
>>>> +     * call site.
>>>> +     *
>>>> +     * NOTE: This code will need to be revisited when kernel CET gets
>>>> +     *       implemented.
>>>> +     */
>>>> +    if (data->ret) {
>>>> +        regs->sp -= sizeof(long);
>>>> +        *(unsigned long *)regs->sp = data->ret;
>>>> +    }
>> 
>> You can’t do this.  Depending on the alignment of the old RSP, which
>> is not guaranteed, this overwrites regs->cs.  IRET goes boom.
> 
> I don't get it; can you spell that out?
> 
> The way I understand it is that we're at a location where a "E8 - Near
> CALL" instruction should be, and thus RSP should be the regular kernel
> stack, and the above simply does "PUSH ret", which is what that CALL
> would've done too.
> 

int3 isn’t IST anymore, so the int3 instruction conditionally subtracts 8 from RSP and then pushes SS, etc. So my email was obviously wrong wrt “cs”, but you’re still potentially overwriting the int3 IRET frame.