linux-kernel - Re: [PATCH 1/2] x86: Fix kernel panic when booting with XD disabled in uEFI firmware

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Message-ID: <20151208141946.GD27180@pd.tnic>
Date:	Tue, 8 Dec 2015 15:19:46 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Matt Fleming <matt@...eblueprint.co.uk>,
	Kees Cook <keescook@...omium.org>
Cc:	Kosuke Tatsukawa <tatsu@...jp.nec.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86: Fix kernel panic when booting with XD disabled
 in uEFI firmware

On Tue, Dec 08, 2015 at 12:25:57PM +0000, Matt Fleming wrote:
> On Mon, 07 Dec, at 11:10:43PM, Kosuke Tatsukawa wrote:
> > 
> > Thank you pointing that out.
> > 
> > linux-4.4-rc3 booted without a problem on a real server even with XD
> > turned off by the firmware.  I didn't notice this before because I was

The aforementioned patch reenables NX.

> Borislav, what do you think about stripping PAGE_NX from 'page_flags'
> in kernel_map_pages_in_pgd() if NX isn't supported, rather than
> returning EINVAL? At least that way EFI runtime services would still
> work.

I guess we can - I mean, I don't see what can go wrong more when
allowing the kernel to execute even NX UEFI regions. Maybe easier
generation of "gadgets" in the ROP sense ...

On a related node, I'm very sceptical of the existence of this "noexec"
chicken bit, if you ask me. It is a really bad idea, security-wise, to
disable NX. Is there even a valid use case to disable NX?

Because if not, I'd vote for removing that chicken bit or at least
taining the kernel with

	add_taint(TAINT_USER_MORON, ... );

Kees, has this NX disabling practice come up in the past, per chance... ?

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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