linux-kernel - Re: [PATCH] kasan, module, vmalloc: rework shadow allocation for modules

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Date:	Fri, 20 Feb 2015 10:47:16 +0300
From:	Andrey Ryabinin <a.ryabinin@...sung.com>
To:	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] kasan, module,
 vmalloc: rework shadow allocation for modules

On 02/20/2015 03:15 AM, Rusty Russell wrote:
> Andrey Ryabinin <a.ryabinin@...sung.com> writes:
>> On 02/19/2015 02:10 AM, Rusty Russell wrote:
>>> This is not portable.  Other archs don't use vmalloc, or don't use
>>> (or define) MODULES_VADDR.  If you really want to hook here, you'd
>>> need a new flag (or maybe use PAGE_KERNEL_EXEC after an audit).
>>>
>>
>> Well, instead of explicit (addr >= MODULES_VADDR && addr < MODULES_END)
>> I could hide this into arch-specific function: 'kasan_need_to_allocate_shadow(const void *addr)'
>> or make make all those functions weak and allow arch code to redefine them.
> 
> That adds another layer of indirection.  And how would the caller of
> plain vmalloc() even know what to return?
> 

I think I don't understand what do you mean here. vmalloc() callers shouldn't know
anything about kasan/shadow.

You were concerned that this patch is not portable, so I suggested to hide arch specific
part in arch code. That's it.

>>> Thus I think modifying the callers is the better choice.
>>>
>>
>> I could suggest following (though, I still prefer 'modifying vmalloc' approach):
>>   * In do_init_module(), instead of call_rcu(&freeinit->rcu, do_free_init);
>>     use synchronyze_rcu() + module_memfree(). Of course this will be
>>   under CONFIG_KASAN.
> 
> But it would be slow, and a disparate code path, which is usually a bad
> idea.
> 
>>     As you said there other module_memfree() users, so what if they will decide
>>     to free memory in atomic context?
> 
> Hmm, how about a hybrid:
> 
> 1) Add kasan_module_alloc(p, size) after module alloc as your original.
> 2) Hook into vfree(), and ignore it if you can't find the map.
> 

That should work, but it looks messy IMO.

> Or is the latter too expensive?
> 

Not sure whether this will be too expensive or not,
but definitely more expensive than simple (addr >= MODULES_VADDR && addr < MODULES_END) check.


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