linux-hardening - Re: KFENCE: included in x86 defconfig?

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Date: Thu, 8 Feb 2024 11:55:17 +0100
From: Borislav Petkov <bp@...en8.de>
To: Marco Elver <elver@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, Matthieu Baerts <matttbe@...nel.org>,
	Alexander Potapenko <glider@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
	Netdev <netdev@...r.kernel.org>, linux-hardening@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: KFENCE: included in x86 defconfig?

On Thu, Feb 08, 2024 at 08:47:37AM +0100, Marco Elver wrote:
> That's a good question, and I don't have the answer to that - maybe we
> need to ask Linus then.

Right, before that, lemme put my user hat on.

> We could argue that to improve memory safety of the Linux kernel more
> rapidly, enablement of KFENCE by default (on the "big" architectures
> like x86) might actually be a net benefit at ~zero performance
> overhead and the cost of 2 MiB of RAM (default config).

What about its benefit?

I haven't seen a bug fix saying "found by KFENCE" or so but that doesn't
mean a whole lot.

The more important question is would I, as a user, have a way of
reporting such issues, would those issues be taken seriously and so on.

We have a whole manual about it:

Documentation/admin-guide/reporting-issues.rst

maybe the kfence splat would have a pointer to that? Perhaps...

Personally, I don't mind running it if it really is a ~zero overhead
KASAN replacement. Maybe as a preliminary step we should enable it on
devs machines who know how to report such things.

/me goes and enables it in a guest...

[    0.074294] kfence: initialized - using 2097152 bytes for 255 objects at 0xffff88807d600000-0xffff88807d800000

Guest looks ok to me, no reports.

What now? :-)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette