linux-kernel - Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

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Date:	Wed, 25 Nov 2015 12:05:25 +0100
From:	"PaX Team" <pageexec@...email.hu>
To:	kernel-hardening@...ts.openwall.com,
	Mathias Krause <minipli@...glemail.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86-ml <x86@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-arch@...r.kernel.org, Emese Revfy <re.emese@...il.com>
Subject: Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

On 25 Nov 2015 at 10:13, Mathias Krause wrote:

> I myself had some educating experience seeing my machine triple fault
> when resuming from a S3 sleep. The root cause was a variable that was
> annotated __read_only but that was (unnecessarily) modified during CPU
> bring-up phase. Debugging that kind of problems is sort of a PITA, you
> could imagine.

actually the kernel could silently recover from this given how the
page fault handler could easily determine that the fault address fell
into the data..read_only section and just silently undo the read-only
property, log the event to dmesg and retry the faulting access.

> So, prior extending the usage of the __read_only annotation some
> toolchain support is needed. Maybe a gcc plugin that'll warn/error on
> code that writes to such a variable but is not __init itself.

this is exactly what i suggested earlier in the constify thread ;).
note that this will produce false positives because __init* annotations
are not propagated everywhere they could be.

> The initify and checker plugins from the PaX patch might be worth to
> look at for that purpose, as they're doing similar things already.

one of our plans for initify is to add the discovery and propagation
of _init* annotations as well, it'd not only fix the false positives
mentioned above but also help reduce the kernel size (code/data/rodata).

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