Re: [PATCH v2 5/5] arm64: add KASan support

[Vladimir Murzin <vladimir.murzin@arm.com>]

On 24/08/15 15:15, Andrey Ryabinin wrote:
> 2015-08-24 16:45 GMT+03:00 Linus Walleij <linus.walleij@linaro.org>:
>> On Mon, Aug 24, 2015 at 3:15 PM, Russell King - ARM Linux
>> <linux@arm.linux.org.uk> wrote:
>>> On Tue, Jul 21, 2015 at 11:27:56PM +0200, Linus Walleij wrote:
>>>> On Tue, Jul 21, 2015 at 4:27 PM, Andrey Ryabinin <a.ryabinin@samsung.com> wrote:
>>>>
>>>>> I used vexpress. Anyway, it doesn't matter now, since I have an update
>>>>> with a lot of stuff fixed, and it works on hardware.
>>>>> I still need to do some work on it and tomorrow, probably, I will share.
>>>>
>>>> Ah awesome. I have a stash of ARM boards so I can test it on a
>>>> range of hardware once you feel it's ready.
>>>>
>>>> Sorry for pulling stuff out of your hands, people are excited about
>>>> KASan ARM32 as it turns out.
>>>
>>> People may be excited about it because it's a new feature, but we really
>>> need to consider whether gobbling up 512MB of userspace for it is a good
>>> idea or not.  There are programs around which like to map large amounts
>>> of memory into their process space, and the more we steal from them, the
>>> more likely these programs are to fail.
>>
>> I looked at some different approaches over the last weeks for this
>> when playing around with KASan.
>>
>> It seems since KASan was developed on 64bit systems, this was
>> not much of an issue for them as they could take their shadow
>> memory from the vmalloc space.
>>
>> I think it is possible to actually just steal as much memory as is
>> needed to cover the kernel, and not 1/8 of the entire addressable
>> 32bit space. So instead of covering all from 0x0-0xffffffff
>> at least just MODULES_VADDR thru 0xffffffff should be enough.
>> So if that is 0xbf000000-0xffffffff in most cases, 0x41000000
>> bytes, then 1/8 of that, 0x8200000, 130MB should be enough.
>> (Andrey need to say if this is possible.)
>>
> 
> Yes, ~130Mb (3G/1G split) should work. 512Mb shadow is optional.
> The only advantage of 512Mb shadow is better handling of user memory
> accesses bugs
> (access to user memory without copy_from_user/copy_to_user/strlen_user etc API).
> In case of 512Mb shadow we could to not map anything in shadow for
> user addresses, so such bug will
> guarantee  to crash the kernel.
> In case of 130Mb, the behavior will depend on memory layout of the
> current process.
> So, I think it's fine to keep shadow only for kernel addresses.

Another option would be having "sparse" shadow memory based on page
extension. I did play with that some time ago based on ideas from
original v1 KASan support for x86/arm - it is how 614be38 "irqchip:
gic-v3: Fix out of bounds access to cpu_logical_map" was caught.
It doesn't require any VA reservations, only some contiguous memory for
the page_ext itself, which serves as indirection level for the 0-order
shadow pages.
In theory such design can be reused by others 32-bit arches and, I
think, nommu too. Additionally, the shadow pages might be movable with
help of driver-page migration patch series [1].
The cost is obvious - performance drop, although I didn't bother
measuring it.

[1] https://lwn.net/Articles/650917/

Cheers
Vladimir

> 
>> That will probably miss some usecases I'm not familiar with, where
>> the kernel is actually executing something below 0xbf000000...
>>
>> I looked at taking memory from vmalloc instead, but ran into
>> problems since this is subject to the highmem split and KASan
>> need to have it's address offset at compile time. On
>> Ux500 I managed to remove all the static maps and steal memory
>> from the top of the vmalloc area instead of the beginning, but
>> that is probably not generally feasible.
>>
>> I suspect you have better ideas than what I can come up
>> with though.
>>
>> Yours,
>> Linus Walleij
> 
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