linux-kernel - Re: [PATCH 11/15] static_call: Add inline static call infrastructure

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Date:   Mon, 10 Jun 2019 18:33:26 +0000
From:   Nadav Amit <namit@...are.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.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 Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 11/15] static_call: Add inline static call infrastructure

> On Jun 10, 2019, at 10:19 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> On Fri, Jun 07, 2019 at 10:37:56AM +0200, Peter Zijlstra wrote:
>>>> +}
>>>> +
>>>> +static int static_call_module_notify(struct notifier_block *nb,
>>>> +				     unsigned long val, void *data)
>>>> +{
>>>> +	struct module *mod = data;
>>>> +	int ret = 0;
>>>> +
>>>> +	cpus_read_lock();
>>>> +	static_call_lock();
>>>> +
>>>> +	switch (val) {
>>>> +	case MODULE_STATE_COMING:
>>>> +		module_disable_ro(mod);
>>>> +		ret = static_call_add_module(mod);
>>>> +		module_enable_ro(mod, false);
>>> 
>>> Doesn’t it cause some pages to be W+X ?
> 
> How so?
> 
>>> Can it be avoided?
>> 
>> I don't know why it does this, jump_labels doesn't seem to need this,
>> and I'm not seeing what static_call needs differently.
> 
> I forgot why I did this, but it's probably for the case where there's a
> static call site in module init code.  It deserves a comment.
> 
> Theoretically, jump labels need this to.
> 
> BTW, there's a change coming that will require the text_mutex before
> calling module_{disable,enable}_ro().

I think that eventually, the most secure flow is for the module executable
to be write-protected immediately after the module signature is checked and
then use text_poke() to change the code without removing the
write-protection in such manner.

Ideally, these pieces of code (module signature check and static-key/call
mechanisms) would somehow be isolated.

I wonder whether static-calls in init-code cannot just be avoided. They
would most likely introduce more overhead in patching than they would save
in execution time.