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

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Message-ID: <AF3846D0-01F0-4A42-AEB6-09B0902A659C@vmware.com>
Date:   Fri, 7 Jun 2019 16:35:42 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     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>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 11/15] static_call: Add inline static call infrastructure

> On Jun 7, 2019, at 1:37 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Thu, Jun 06, 2019 at 10:24:17PM +0000, Nadav Amit wrote:
> 
>>> +static void static_call_del_module(struct module *mod)
>>> +{
>>> +	struct static_call_site *start = mod->static_call_sites;
>>> +	struct static_call_site *stop = mod->static_call_sites +
>>> +					mod->num_static_call_sites;
>>> +	struct static_call_site *site;
>>> +	struct static_call_key *key, *prev_key = NULL;
>>> +	struct static_call_mod *site_mod;
>>> +
>>> +	for (site = start; site < stop; site++) {
>>> +		key = static_call_key(site);
>>> +		if (key == prev_key)
>>> +			continue;
>>> +		prev_key = key;
>>> +
>>> +		list_for_each_entry(site_mod, &key->site_mods, list) {
>>> +			if (site_mod->mod == mod) {
>>> +				list_del(&site_mod->list);
>>> +				kfree(site_mod);
>>> +				break;
>>> +			}
>>> +		}
>>> +	}
>> 
>> I think that for safety, when a module is removed, all the static-calls
>> should be traversed to check that none of them calls any function in the
>> removed module. If that happens, perhaps it should be poisoned.
> 
> We don't do that for normal indirect calls either.. I suppose we could
> here, but meh.
> 
>>> +}
>>> +
>>> +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 ? 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.
> 
>>> +		if (ret) {
>>> +			WARN(1, "Failed to allocate memory for static calls");
>>> +			static_call_del_module(mod);
>> 
>> If static_call_add_module() succeeded in changing some of the calls, but not
>> all, I don’t think that static_call_del_module() will correctly undo
>> static_call_add_module(). The code transformations, I think, will remain.
> 
> Hurm, jump_labels has the same problem.
> 
> I wonder why kernel/module.c:prepare_coming_module() doesn't propagate
> the error from the notifier call. If it were to do that, I think we'll
> abort the module load and any modifications get lost anyway.

This might be a security problem, since it can leave indirect branches,
which are susceptible to Spectre v2, in the code.