Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()

[Alexander Popov]

Hello Linus,

Thanks for your reply (despite some strong words).

On 05.03.2018 23:15, Linus Torvalds wrote:
> This is the first I see of any of this, it was apparently not actually
> posted to lkml or anything like that.

I described that below.

> Honestly, what I see just makes me go "this is security-masturbation".

Let me quote the cover letter of this patch series.

STACKLEAK (initially developed by PaX Team):
 - reduces the information that can be revealed through kernel stack leak bugs;
 - blocks some uninitialized stack variable attacks (e.g. CVE-2017-17712,
CVE-2010-2963);
 - introduces some runtime checks for kernel stack overflow detection. It blocks
the Stack Clash attack against the kernel.

So it seems to be a useful feature.

> It doesn't actually seem to help *find* bugs at all. As such, it's
> another "paper over and forget" thing that just adds fairly high
> overhead when it's enabled.

The cover letter also contains the information about the performance impact.
It's 0.6% on the compiling the kernel (with Ubuntu config) and approx 4% on a
very intensive hackbench test.

> I'm NAK'ing it sight-unseen (see above) just because I'm tired of
> these kinds of pointless things that don't actually strive to improve
> on the kernel, just add more and more overhead for nebulous "things
> may happen", and that just make the code uglier.
> 
> Why wasn't it even posted to lkml?

That's my mistake. I started to learn that feature 9 month ago, just before
Qualys published the Stack Clash attack (which is blocked by STACKLEAK). I sent
first WIP versions to a short list of people (and had a lot of feedback to work
with). But later unfortunately I didn't adjust the list of recipients.

That was not done intentionally.

> And why isn't the focus of security people on tools to _analyse_ and
> find problems?

You know, I like KASAN and kernel fuzzing as well:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=82f2341c94d270421f383641b7cd670e474db56b

Best regards,
Alexander