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Date:   Tue, 12 Apr 2022 17:16:56 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Gerber <peter@...tirary.ch>
Cc:     kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org
Subject: Re: Kernel Self Protection Project: slub_debug=ZF

On Sun, Apr 10, 2022 at 09:34:17PM +0200, Peter Gerber wrote:
> Hello,
> 
> The Kernel Self Protection Project, on their Recommended Settings [1] page,
> suggests the following:
> 
> # Enable SLUB redzoning and sanity checking (slow; requires
> CONFIG_SLUB_DEBUG=y above).
> slub_debug=ZF
> 
> On recent kernels, I see the following in dmesg when this option is set:
> 
> **********************************************************
> **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
> **                                                      **
> ** This system shows unhashed kernel memory addresses   **
> ** via the console, logs, and other interfaces. This    **
> ** might reduce the security of your system.            **
> **                                                      **
> ** If you see this message and you are not debugging    **
> ** the kernel, report this immediately to your system   **
> ** administrator!                                       **
> **                                                      **
> **   NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE NOTICE   **
> **********************************************************
> 
> A bit of digging tells me that this is caused by "slub: force on
> no_hash_pointers when slub_debug is enabled" [2]. Assuming the performance
> impact is acceptable, is this option still recommend? Should there perhaps
> be a way to explicitly disable no_hash_pointers (e.g. via
> no_hash_pointers=off)?

Eww, that's not good at all. Would you be willing to write a patch to
decouple this again?

I think the primary issue is that these "debug" modes aren't exclusively
used for debugging, and exposing the unhashed pointer is directly in
conflict with the non-debug use case. :P

-Kees

> 
> Regards,
> 
> Peter
> 
> [1]: https://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project/Recommended_Settings
> [2]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=792702911f581f7793962fbeb99d5c3a1b28f4c3

-- 
Kees Cook

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