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

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Date:   Mon, 10 Jun 2019 13:42:39 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nadav Amit <namit@...are.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 Mon, Jun 10, 2019 at 06:33:26PM +0000, Nadav Amit wrote:
> > 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.

It's a valid question.  Are any tracepoints called from module init?  Or
-- thinking ahead -- are there any pv calls from module init?  That
might be plausible.

-- 
Josh