linux-kernel - Re: [PATCH] arm64: make CONFIG_DEBUG

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Message-ID: <20160128145900.GN17123@leverpostej>
Date:	Thu, 28 Jan 2016 14:59:01 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Kees Cook <keescook@...omium.org>
Cc:	David Brown <david.brown@...aro.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Mathias Krause <minipli@...glemail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	PaX Team <pageexec@...email.hu>,
	Emese Revfy <re.emese@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Marc Zyngier <marc.zyngier@....com>,
	yalin wang <yalin.wang2010@...il.com>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	Yang Shi <yang.shi@...aro.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Laura Abbott <lauraa@...eaurora.org>,
	"Suzuki K. Poulose" <suzuki.poulose@....com>,
	Steve Capper <steve.capper@...aro.org>,
	Jeremy Linton <jeremy.linton@....com>,
	Mark Salter <msalter@...hat.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: make CONFIG_DEBUG_RODATA non-optional

On Thu, Jan 28, 2016 at 06:06:53AM -0800, Kees Cook wrote:
> On Thu, Jan 28, 2016 at 3:06 AM, Mark Rutland <mark.rutland@....com> wrote:
> > One thing I would like to do is to avoid the need for fixup_executable
> > entirely, by mapping the kernel text RO from the outset. However, that
> > requires rework of the alternatives patching (to use a temporary RW
> > alias), and I haven't had the time to look into that yet.
> 
> This makes perfect sense for the rodata section, but the (future)
> postinit_rodata section we'll still want to mark RO after init
> finishes. x86 and ARM cheat by marking both RO after init, and they
> don't have to pad sections. parisc will need to solve this too.

Depending on how many postinit_rodata variables there are, we might be
able to drop those in .rodata, have them RO always, and initialise them
via a temporary RW alias (e.g. something in the vmalloc area).

The only requirement for that is that we use a helper to initialise any
__postinit_ro variables via a temporary RW alias, e.g.

#define SET_POST_INIT_RO(ptr, v) ({ 		\\
	typeof(ptr) __ptr_rw = (ptr)		\\
	BUG_ON(initcalls_done);			\\
	__ptr_rw = create_rw_alias(__ptr);	\\
	__ptr_rw = v;				\\
	destroy_rw_alias(__ptr_rw);		\\
})

...

__postinit_ro void *thing;

void __init some_init_func(void) {
	void *__thing = some_ranodomized_allocator();
	SET_POSTINIT_RO(thing, thing);
}


Mark.