linux-kernel - Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early

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Message-Id: <D67F7C19-DF59-4405-A5A3-B56C0E491B41@amacapital.net>
Date:   Mon, 5 Nov 2018 11:03:49 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Nadav Amit <namit@...are.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init



> On Nov 5, 2018, at 9:49 AM, Nadav Amit <namit@...are.com> wrote:
> 
> From: Andy Lutomirski
> Sent: November 5, 2018 at 5:22:32 PM GMT
>> To: Peter Zijlstra <peterz@...radead.org>
>> Cc: Nadav Amit <namit@...are.com>, Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org, x86@...nel.org, H. Peter Anvin <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, Kees Cook <keescook@...omium.org>, Dave Hansen <dave.hansen@...el.com>, Masami Hiramatsu <mhiramat@...nel.org>
>> Subject: Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init
>> 
>> 
>> 
>>>> On Nov 5, 2018, at 6:09 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>>> 
>>>> On Fri, Nov 02, 2018 at 04:29:41PM -0700, Nadav Amit wrote:
>>>> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
>>>> index aac0c1f7e354..367c1d0c20a3 100644
>>>> --- a/arch/x86/kernel/jump_label.c
>>>> +++ b/arch/x86/kernel/jump_label.c
>>>> @@ -52,7 +52,13 @@ static void __ref __jump_label_transform(struct jump_entry *entry,
>>>>  jmp.offset = jump_entry_target(entry) -
>>>>           (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
>>>> 
>>>> -    if (early_boot_irqs_disabled)
>>>> +    /*
>>>> +     * As long as we are in early boot, we can use text_poke_early(), which
>>>> +     * is more efficient: the memory was still not marked as read-only (it
>>>> +     * is only marked after poking_init()). This also prevents us from using
>>>> +     * text_poke() before poking_init() is called.
>>>> +     */
>>>> +    if (!early_boot_done)
>>>>      poker = text_poke_early;
>>>> 
>>>>  if (type == JUMP_LABEL_JMP) {
>>> 
>>> It took me a while to untangle init/maze^H^Hin.c... but I think this
>>> is all we need:
>>> 
>>> diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
>>> index aac0c1f7e354..ed5fe274a7d8 100644
>>> --- a/arch/x86/kernel/jump_label.c
>>> +++ b/arch/x86/kernel/jump_label.c
>>> @@ -52,7 +52,12 @@ static void __ref __jump_label_transform(struct jump_entry *entry,
>>>  jmp.offset = jump_entry_target(entry) -
>>>           (jump_entry_code(entry) + JUMP_LABEL_NOP_SIZE);
>>> 
>>> -    if (early_boot_irqs_disabled)
>>> +    /*
>>> +     * As long as we're UP and not yet marked RO, we can use
>>> +     * text_poke_early; SYSTEM_BOOTING guarantees both, as we switch to
>>> +     * SYSTEM_SCHEDULING before going either.
>>> +     */
>>> +    if (system_state == SYSTEM_BOOTING)
>>>      poker = text_poke_early;
>>> 
>>>  if (type == JUMP_LABEL_JMP) {
>> 
>> Can we move this logic into text_poke() and get rid of text_poke_early()?
> 
> This will negatively affect poking of modules doing module loading, e.g.,
> apply_paravirt(). This can be resolved by keeping track when the module is
> write-protected and giving a module parameter to text_poke(). Does it worth
> the complexity?

Probably not.

OTOH, why does alternative patching need text_poke() at all?  Can’t it just write to the text?

> 
>> FWIW, alternative patching was, at some point, a significant fraction of
>> total boot time in some cases. This was probably mostly due to unnecessary
>> sync_core() calls. Although I think this was reported on a VM, and
>> sync_core() used to be *extremely* expensive on a VM, but that’s fixed
>> now, and it even got backported, I think.
>> 
>> (Hmm. Maybe we can also make jump label patching work in early boot, too!)
> 
> It may be possible to resolve the dependencies between poking_init() and the
> other *_init(). I first considered doing that, yet, it makes the code very
> fragile, and I don’t see the value in getting rid of text_poke_early() from
> security or simplicity point of views. Let me know if you think otherwise.
> 
> Regards,
> Nadav