Warn users about machines with non-working WP bit

Warn users about machines with non-working WP bit

[Pavel Machek]

Hi!

This might be good idea, as those machines are not safe for multiuser
systems.

--- clean.2.5/arch/i386/mm/init.c	Sun Mar 10 20:06:31 2002
+++ linux/arch/i386/mm/init.c	Mon Mar 11 21:49:14 2002
@@ -383,7 +383,7 @@
 	local_flush_tlb();
 
 	if (!boot_cpu_data.wp_works_ok) {
-		printk("No.\n");
+		printk("No (that's security hole).\n");
 #ifdef CONFIG_X86_WP_WORKS_OK
 		panic("This kernel doesn't support CPU's with broken WP. Recompile it for a 386!");
 #endif

									Pavel
-- 
(about SSSCA) "I don't say this lightly.  However, I really think that the U.S.
no longer is classifiable as a democracy, but rather as a plutocracy." --hpa

Re: Warn users about machines with non-working WP bit

[Brian Gerst]

Pavel Machek wrote:
> 
> Hi!
> 
> This might be good idea, as those machines are not safe for multiuser
> systems.
> 
> --- clean.2.5/arch/i386/mm/init.c       Sun Mar 10 20:06:31 2002
> +++ linux/arch/i386/mm/init.c   Mon Mar 11 21:49:14 2002
> @@ -383,7 +383,7 @@
>         local_flush_tlb();
> 
>         if (!boot_cpu_data.wp_works_ok) {
> -               printk("No.\n");
> +               printk("No (that's security hole).\n");
>  #ifdef CONFIG_X86_WP_WORKS_OK
>                 panic("This kernel doesn't support CPU's with broken WP. Recompile it for a 386!");
>  #endif
> 
>                                                                         Pavel

The "bug" is really the lack of a feature present on 486+ cpus.  A 386
will allow the kernel to write to a write-protected user page (but not a
write-protected kernel page).  In user mode, write protect works as it
should.  The kernel works around this by doing extra checks when writing
to user pages (check the *_user() functions).  It is not a security
hole, because if the kernel wasn't compiled with the workaround, it
refuses to boot on those cpus.

--

				Brian Gerst

Re: Warn users about machines with non-working WP bit

[Pavel Machek]

Hi!

> > This might be good idea, as those machines are not safe for multiuser
> > systems.
> > 
> > --- clean.2.5/arch/i386/mm/init.c       Sun Mar 10 20:06:31 2002
> > +++ linux/arch/i386/mm/init.c   Mon Mar 11 21:49:14 2002
> > @@ -383,7 +383,7 @@
> >         local_flush_tlb();
> > 
> >         if (!boot_cpu_data.wp_works_ok) {
> > -               printk("No.\n");
> > +               printk("No (that's security hole).\n");
> >  #ifdef CONFIG_X86_WP_WORKS_OK
> >                 panic("This kernel doesn't support CPU's with broken WP. Recompile it for a 386!");
> >  #endif
> > 
> >                                                                         Pavel
> 
> The "bug" is really the lack of a feature present on 486+ cpus.  A 386
> will allow the kernel to write to a write-protected user page (but not a
> write-protected kernel page).  In user mode, write protect works as it
> should.  The kernel works around this by doing extra checks when writing
> to user pages (check the *_user() functions).  It is not a security

It is, because those checks are racy when clone() is in use. Linus
stated that few times.
								Pavel
-- 
Casualities in World Trade Center: ~3k dead inside the building,
cryptography in U.S.A. and free speech in Czech Republic.