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

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Message-ID: <4AF9D98A.90808@zytor.com>
Date:	Tue, 10 Nov 2009 13:22:18 -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 12:55 PM, Kees Cook wrote:
>> > 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),
> Well, I think thinking that even if they turned on the flag in the BIOS,
> the non-PAE kernel couldn't do anything about it anyway.  But, from your
> example, I see you went with "missing in kernel" anyway.

No, I didn't: in my example, the CPU checks have higher priority than
the kernel feature check.

>> So the logic that makes sense would be:
>>
>> if (!cpu_has_nx) {
> 
> cpu_has_nx is not the same as nx_enabled (due to disable_nx).  Also, why
> doesn't set_nx() use cpu_has_nx?  It seems like it does the check
> manually?  Should that be cleaned up?

Yes, it should be.  set_nx() and check_efer() are doing the same thing,
except in different ways, and they are - IMO - *both* doing something
dumb -- although check_efer() is saner.

Anyway, I forgot the last case, which is NX disabled manually
(disable_nx).  It probably makes sense to make it the lowest priority
message.

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 not supported by kernel\n");
#else
	if (disable_nx)
		printk(KERN_INFO "cpu: NX protection disabled by kernel command line
option\n");
	else
		printk(KERN_INFO "cpu: NX protection active\n");
#endif
}


> How about this?  (Along with the nx_enabled setting in set_nx() for the
> 64-bit and 32-bit+PAE case.)

No, it gives the wrong message for the manually disabled case.

	-hpa
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