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

From: Andrey Ryabinin
Date: Fri Feb 20 2015 - 02:47:30 EST

On 02/20/2015 03:15 AM, Rusty Russell wrote:
> Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> 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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