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

From: Rusty Russell
Date: Mon Feb 23 2015 - 07:02:15 EST

Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx> writes:
> 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.

How else would kasan_need_to_allocate_shadow(const void *addr) work for
architectures which don't have a reserved vmalloc region for modules?

>> 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.

Sure, if that check were portable. If you ever wanted kasan on other
vmalloc addresses it wouldn't work either.

I actually think this pattern is the *simplest* solution for auxilliary
data like kasan.

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