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Date: Thu, 22 Mar 2018 17:14:40 -0700
From: Kees Cook <keescook@...omium.org>
To: Daniel Vetter <daniel@...ll.ch>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, David Airlie <airlied@...ux.ie>,
dri-devel <dri-devel@...ts.freedesktop.org>, Sean Paul <seanpaul@...omium.org>,
Cihangir Akturk <cakturk@...il.com>, Ingo Molnar <mingo@...nel.org>,
Krzysztof Kozlowski <krzk@...nel.org>, Tom Lendacky <thomas.lendacky@....com>,
Eyal Itkin <eyalit@...ckpoint.com>, Daniel Micay <danielmicay@...il.com>
Subject: Re: [PATCH] drm: udl: Properly check framebuffer mmap offsets
On Thu, Mar 22, 2018 at 2:54 AM, Daniel Vetter <daniel@...ll.ch> wrote:
> On Thu, Mar 22, 2018 at 9:03 AM, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> wrote:
>> On Thu, Mar 22, 2018 at 07:59:59AM +0100, Daniel Vetter wrote:
>>> Does anyone working on overflow-proof integers? That would make a lot of
>>> this code so much simpler if we could just ask the compiler to carry the
>>> oferflow bit around for a given expression and then check that and bail
>>> with -EINVAL.
>>
>> That would be nice, but no, I don't think that's part of any C standard
>> work that I have heard of :(
>
> Well we have refcount_t already, stitching something together that
> would work and not suck too badly with performance should be possible.
> But yeah direct compiler support would be better (and would allow
> optimizing the carry flag checks I guess). I kinda hoped Kees&team
> would be working on this eventually.
refcount_t could be used if it happens to match the needed semantics.
> Adding Kees+kernel-hardening, maybe he'll grow fond of this :-)
Yeah, general integer overflow is on the list of things to get fixed
in the kernel. It's a bit of a long road, though.
Clang has -fsanitize=integer (and sub-options) which could be added
for specific object or trees:
https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#ubsan-checks
GCC seems to only support manual marking of overflow detections:
https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html
grsecurity/PaX has a gcc plugin for overflow detection, though it
hasn't been upstreamed and comes with various caveats:
http://forums.grsecurity.net/viewtopic.php?f=7&t=3043
https://github.com/ephox-gcc-plugins/size_overflow
-Kees
--
Kees Cook
Pixel Security
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