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

[Nadav Amit]

> On Jun 7, 2019, at 1:37 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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);
>> 
>> Doesnât 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 donât 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.