Re: [PATCH v3 2/7] x86/jump_label: Use text_poke_early() during early_init

[Nadav Amit]

From: Andy Lutomirski
Sent: November 5, 2018 at 5:22:32 PM GMT
> To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Nadav Amit <namit@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, x86@xxxxxxxxxx, H. Peter Anvin <hpa@xxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxx>, Kees Cook <keescook@xxxxxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxx>, Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> 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@xxxxxxxxxxxxx> 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?

> 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 thatâs 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 donât 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