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

[Kees Cook]

On Tue, Dec 8, 2015 at 6:19 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> 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, ... );

If we add this for not-nx, I would like to add it for not-rodata too.

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

I've never seen anyone actually use it. I was asked to include it out
of fear of some kind of rogue imagined CPU configuration that mixed NX
and non-NX capable CPUs in a single machine where the forced NX
re-enablement code would cause problems. As you might imagine, I'm not
aware of this case ever being an issue. ;)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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