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

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

On Tue, Nov 10, 2009 at 09:12:44AM -0800, H. Peter Anvin wrote:
> On 11/10/2009 08:57 AM, Kees Cook wrote:
> > 
> > 64-bit does not set nx_enabled to "1" by default anywhere.  And setting
> > the default to 1 in check_efer() seemed out of place to me.
> > 
> 
> If it doesn't set nx_enabled anywhere, you'll end up with the message
> 
> +	printk(KERN_NOTICE "Notice: NX (Execute Disable) protection "
> +	       "missing in CPU or disabled in BIOS!\n");
> 
> which seems obviously wrong... no?

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.


My patch seeks to do two things:
 - report the "active" message on 64-bit (if not disabled in check_efer())
 - report "zomg NX missing" for all cases when PAE is available to the CPU.

To implement the first, the set_nx() routine for non-PAE 32-bit and all
64-bit needs to have nx_enabled set correctly (which is done in the second
set_nx() that was empty in arch/x86/mm/setup_nx.c).

To implement the second, the following logic is then added to the "if
(nx_enabled)" case in arch/x86/mm/init.c as an "else":

        else if (cpu_has_pae)
#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