lkml.org 
[lkml]   [2009]   [Nov]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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
}


\
 
 \ /
  Last update: 2009-11-10 21:05    [W:0.080 / U:0.720 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site