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Message-ID: <CAKv+Gu_ss7a4putFyStN6RaK_OM3fFezB3MhF1O1+9=0RaupZA@mail.gmail.com>
Date: Thu, 3 Aug 2017 23:42:49 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Kees Cook <keescook@...omium.org>
Cc: Arnd Bergmann <arnd@...db.de>, Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC/RFT PATCH] gcc-plugins: force initialize auto variables
whose addresses are taken
On 3 August 2017 at 23:14, Kees Cook <keescook@...omium.org> wrote:
> On Thu, Aug 3, 2017 at 11:27 AM, Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
>> On 3 August 2017 at 05:35, Kees Cook <keescook@...omium.org> wrote:
>>> On Thu, Jul 6, 2017 at 4:16 PM, Kees Cook <keescook@...omium.org> wrote:
>>>> On Thu, Jul 6, 2017 at 2:44 PM, Kees Cook <keescook@...omium.org> wrote:
>>>>> On Thu, Jul 6, 2017 at 4:09 AM, Arnd Bergmann <arnd@...db.de> wrote:
>>>>>> On Thu, Jul 6, 2017 at 12:13 PM, Ard Biesheuvel
>>>>>> <ard.biesheuvel@...aro.org> wrote:
>>>>>>> To prevent leaking stack contents in cases where it is not possible
>>>>>>> for the compiler to figure out whether an automatic variable has been
>>>>>>> initialized or not, add a plugin that forcibly initializes all automatic
>>>>>>> variables of struct/union types if their address is taken at any point.
>>>>>>>
>>>>>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>>>>>
>>>>> Ard, I'd be curious what you see for "size" difference between builds
>>>>> and if it's visible with hackbench or other things?
>>>>
>>>> Hm, not all that bad on the size front:
>>>>
>>>> text data bss dec
>>>> hex filename
>>>> 10950705 5592525 13955072 30498302 1d15dfe vmlinux
>>>> 11048035 5592365 13955072 30595472 1d2d990
>>>> vmlinux.initautobyref
>>>>
>>>> And yes, as expected, wow there are a lot of notices in verbose mode. ;)
>>>>
>>>> My pet favorite, from the NAKed patch I sent forever ago, is covered
>>>> (as expected):
>>>>
>>>> net/socket.c: In function ‘SYSC_getsockname’:
>>>> net/socket.c:1605:26: note: auto variable will be forcibly initialized
>>>> struct sockaddr_storage address;
>>>> ^~~~~~~
>>>
>>> While this was an RFC, it seems to work well and, as Daniel mentioned,
>>> provides another benchmark for future optimizations of this kind of
>>> protection. Besides the COMPILE_TEST change already discussed, any
>>> other changes or objections before I carry this in -next?
>>>
>>
>> Sounds reasonable to me.
>
> Actually, I just looked at the diff between structleak and
> initautobyref, and it's essentially 1 test (and the removal of all the
> __user-detection code):
>
> - /* if the type is of interest, examine the variable */
> - if (TYPE_USERSPACE(type))
> + /* initialize the variable if its address is taken */
> + if (TREE_ADDRESSABLE (var))
>
> Perhaps instead of a whole new plugin, could we just add the
> functionality to the existing structleak plugin as a Kconfig option?
> Like maybe CONFIG_GCC_PLUGIN_STRUCTLEAK_TAKEN?
>
Yeah, it is mostly the same code. As you know, it was mainly intended
as a PoC but given the interest to merge this functionality for real,
I will do another pass and try to incorporate it more cleanly.
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