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

[Borislav Petkov]

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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