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

[Kees Cook <kees.cook@canonical.com>]

On Tue, Nov 10, 2009 at 10:53:33AM -0800, H. Peter Anvin wrote:
> On 11/10/2009 09:46 AM, Kees Cook wrote:
> > 
> > The kernel as-is does not set nx_enabled for 64-bit, so this message is
> > skipped completely:
> > 
> >         if (nx_enabled)
> >                 printk(KERN_INFO "NX (Execute Disable) protection: active\n");
> > 
> > The only time this printk is shown is on 32-bit with PAE (with NX).
> > There is no "else" currently.
> > 
> 
> The structure you have is:
> 
> 	if (nx_enabled)
> 	else if (cpu_has_pae)
> 
> The test for cpu_has_pae is unconditional (you only #ifdef the message)
> -- in fact, this should cause a compile-time error on 64 bits:
> 
> #undef  cpu_has_pae
> #define cpu_has_pae             ___BUG___

This is fun.  CONFIG_X86_PAE isn't defined for 64-bit, and using
cpu_has_pae on 64-bit is considered a bug.  :)

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.


this looks ugly, but does it:

#if defined(CONFIG_X86_32)
	else if (cpu_has_pae)
#else
	else
#endif
#if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
		/* PAE kernel, PAE CPU, without NX */
		printk(KERN_NOTICE "Notice: NX (Execute Disable) protection "
		       "missing in CPU or disabled in BIOS!\n");
#else
		/* 32bit non-PAE kernel, PAE CPU */
		printk(KERN_NOTICE "Notice: NX (Execute Disable) protection "
		       "cannot be enabled: non-PAE kernel!\n");
#endif


-- 
Kees Cook
Ubuntu Security Team