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Date: Fri, 19 Dec 2014 22:41:28 +0100
From: Mathias Krause <>
Subject: Re: How GNU/Linux distros deal with offset2lib attack?

On 18 December 2014 at 22:50, Greg KH <> wrote:
> On Thu, Dec 18, 2014 at 11:36:03AM +0100, Mathias Krause wrote:
>> On 18 December 2014 at 10:35, Amos Jeffries <> wrote:
>> > On 18/12/2014 9:24 p.m., Lionel Debroux wrote:
>> All wrong. As Lionel wrote, the code assigns the variable before
>> reading it. So no data is meant to persist between multiple calls to
>> this function. However, if max8925_probe() gets called concurrently,
>> the 'chip' pointer may change beneath one of the threads -- not good.
>> So this is clearly a fix.
> But that function can not be called concurrently, so this doesn't
> matter.

Thanks for clarifying this. Still, it's worth to fix this, no? Even if
this is not a bug in a sense that it would be exploitable in any way,
it's obfuscating things.

>> >  People using PaX code are trusting that they have done the analysis,
>> Obviously they did.
> Someone got it wrong :)

Fixing obfuscated code is wrong -- got it.

>> > but that very code not being in mainline means there is possibly no
>> > hard proof of that.
>> You're wrong, again. No-one submitted the fix to LKML, that's the reason.
> And if they did, they would have gotten the review I just gave.

So you advocate for leaving the 'static' in place just because "it's
not a bug"? That's ridiculous!
Can you please point me to the part in Documentation/CodingStyle were
it says obfuscated code is the preferred kernel coding style? Thanks.

>> >> or local structs which are not meant
>> >> to be modified and should therefore probably be made static /
>> >> static const (mainline doesn't use the GCC plugin for
>> >> constification):
>> >> [...]
>> >
>> > Now *that* does just appear to be a gratuitous cleanup / performance
>> > booster. Not security related.
>> Wrong. PaX contains a gcc plugin that does *automatic* constification
>> of eligible structures (structures containing function pointers).
>> That's incompatible with run-time modification of the data structures
>> in question. Therefore this change fixes the incompatibility by making
>> the run-time assignment a compile time constant.
> Ok, that's not very obvious just looking at the patch :)

Lionel mentioning "GCC plugin for constification" might have been a
hint there's more to it ;)

LWN wrote something about it a few years ago:
The comments are worth reading, as the PaX Team explains a few more
things in there.

>> Making structures containing function pointers r/o actually is
>> security related. Read only data structures cannot be abused by memory
>> corruption bugs, e.g., like the exploit for CVE-2013-2094 which
>> overwrites function pointers in ptmx_fops to get code execution. But,
>> well, that's true for PaX only, as write protected kernel r/o data is
>> something mainline only gets when CONFIG_DEBUG_RODATA is set -- a
>> '"Kernel hacking" debug option. Tells much about the state of security
>> philosophy in the mainline kernel...
> Cut it out with your "state of security philosophy" crap please, it
> benifits no one except making you try to feel better about yourself.
> If you wish to help us out with the "state of security" in the kernel,
> please send patches to do so.

Honestly, no, thanks. I won't try to increase the state of security in
the kernel. It's just not worth the pain, IMHO. It's way easier for me
to build upon the PaX / grsecurity patches and contribute security
enhancements there. Those patches already fix quite a few issues and
are dedicated to enhance the state of kernel security. So why should I
step back and try to re-implement things upstream that are already
solved for years (or decades, even) in PaX / grsecurity?

>  Participate in patch review, read all
> 10000 patches that get merged every 2 months and send me git ids to be
> included in stable kernels, or any number of other valid, helpful things
> you could do instead of just pointing at a huge, external, patchset and
> saying that somehow the kernel developers (all 5000 of us I guess) have
> no idea what we are doing.

Don't put words into my mouth I never said. I never said, you guys
have no idea what you're doing. I also never suggested to merge PaX /
grsecurity into mainline. I never will! Just because it would defeat
the purpose of the patch. The developers of PaX and grsecurity would
loose control over there changes made to the kernel, possibly making
those changes bit-rot and weaken each and every kernel release.
No, in fact, I prefer the permanent fork mode of PaX and grsecurity as
it gives me certainty, the guys caring about security actually look
after if there changes are still functioning by solving merge
conflicts themselves. As those changes are spread all over the tree
that won't work with the upstream maintainer model.

It looks like none of the 5000 kernel developers has a strong interest
in security. If just a single one would, why wouldn't that single
person take DEBUG_RODATA out of it's ridiculous location and make it
the default -- only optionally with an inverted option below "Kernel
hacking" to allow debugging crappy drivers. Please tell me. But I'd
guess the answer is that such a change would "fix no bug". Therefore
it "doesn't matter." So, unfortunately, that kernel hardening feature
has to live on as a "debug option" -- sad, very sad, IMHO.


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