linux-kernel - Re: [PATCH v4] [x86] detect and report lack of NX protections

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Date:	Tue, 10 Nov 2009 11:59:35 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <kees.cook@...onical.com>
CC:	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Jan Beulich <jbeulich@...ell.com>,
	Vegard Nossum <vegardno@....uio.no>,
	Yinghai Lu <yinghai@...nel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] [x86] detect and report lack of NX protections

On 11/10/2009 11:43 AM, Kees Cook wrote:
> 
> This is fun.  CONFIG_X86_PAE isn't defined for 64-bit, and using
> cpu_has_pae on 64-bit is considered a bug.  :)
> 

Yeah, it's somewhat obnoxious.  This stuff is a result of the 32- and
64-bit code evolving separately for too long.  All of this could and
should be cleaned up, but it takes a long time.

Either way, you can use the explicit form:

	boot_cpu_has(X86_FEATURE_PAE)

just fine, on any platform.  However, the only case for which this can
be false is for the non-PAE kernel, since the PAE kernels (32 or 64
bits) cannot boot without it.  I have personally never liked the
cpu_has_* shorthand macros, but they're occasionally useful for things
that have to be handled specially on 64 bits.  Unfortunately they have
spread and people seem to think they're the only way.

> Here is the matrix of what I want to see reported about NX at boot time.
> How do you recommend this be implemented?
> 
> kernel  cpu -> |              CPU has PAE              |  CPU lacks PAE  |
>    |           |       CPU has NX  | CPU lacks NX      |                 |
>    V           +-------------------+-------------------+-----------------+
> 32-bit non-PAE | missing in kernel | missing in kernel |    no message   |
>                +-------------------+-------------------+-----------------+
> 32-bit PAE     | active *          | missing in CPU    |    no message   |
>                +-------------------+-------------------+-----------------+
> 64-bit         | active            | missing in CPU    |    impossible   |
>                +-------------------+-------------------+-----------------+
> The box with the "*" is the only message currently reported by the kernel.

The last column should actually be "no message", "impossible", "impossible".

I also think "missing in kernel" is misleading in the 32-bit non-PAE,
no-NX case (as it would imply that another kernel could do something),
and I *really* fail to see why it is in any way different from the "CPU
lacks PAE" case -- which also means no NX.  "Unavailable in CPU" seems
to beat everything.

So the logic that makes sense would be:

if (!cpu_has_nx) {
	/* If the CPU can't do it... */
	printk(KERN_INFO "cpu: NX protection unavailable in CPU\n");
} else  {
#if defined(CONFIG_X86_32) && !defined(CONFIG_X86_PAE)
	/* Non-PAE kernel: NX unavailable */
	printk(KERN_NOTICE "cpu: NX protection missing in kernel\n");
#else
	printk(KERN_INFO "cpu: NX protection active\n");
#endif
}
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