Re: [patch] mark text section read-only

[Coywolf Qi Hunt <coywolf@gmail.com>]

2005/11/12, linux-os (Dick Johnson) <linux-os@analogic.com>:
>
> On Fri, 11 Nov 2005, Coywolf Qi Hunt wrote:
>
> > On Sat, Nov 12, 2005 at 02:57:02AM +0800, Coywolf Qi Hunt wrote:
> >> And we could also mark text section read-only and data/stack section
> >> noexec if NX is supported. But I doubt the whole thing would really
> >> help much. Kill the kernel thread? We can't. We only run into a panic.
> >> Anyway I'd attach a quick patch to mark text section read only in the
> >> next mail.
> >>
> >> If it's ok, I'd add Kconfig support. Comments?
> >
> >
> > Signed-off-by: Coywolf Qi Hunt <qiyong@fc-cn.com>
> > ---
> >
> > diff -pruN 2.6.14-mm2/init/main.c 2.6.14-mm2-cy/init/main.c
> > --- 2.6.14-mm2/init/main.c    2005-11-11 22:34:21.000000000 +0800
> > +++ 2.6.14-mm2-cy/init/main.c 2005-11-12 02:50:45.000000000 +0800
> > @@ -660,6 +660,18 @@ static inline void fixup_cpu_present_map
> > #endif
> > }
> >
> > +void mark_text_ro(void)
> > +{
> > +     unsigned long addr = (unsigned long)&_text;
> > +
> > +     for (; addr < (unsigned long)&_etext; addr += PAGE_SIZE)
> > +             change_page_attr(virt_to_page(addr), 1, PAGE_KERNEL_RO);
> > +
> > +     printk ("Write protecting the kernel text data: %luk\n",
> > +                     (unsigned long)(_etext - _text) >> 10);
> > +     global_flush_tlb();
> > +}
> > +
> > static int init(void * unused)
> > {
> >       lock_kernel();
> > @@ -716,6 +728,7 @@ static int init(void * unused)
> >        */
> >       free_initmem();
> >       unlock_kernel();
> > +     mark_text_ro();
> >       mark_rodata_ro();
> >       system_state = SYSTEM_RUNNING;
> >       numa_default_policy();
> > -
>
>
> Assuming ix86, what is read-only? Certainly the text section needs
> to be marked execute!
>
> So is it:
>         Execute-Only
>         Execute-Only, accessed
>         Execute/Read
>         Execute/Read, accessed
>         Execute-only, conforming
>         Execute-only, conforming, accessed
>         Execute/Read-Only, conforming
>         Execute/Read-Only, conforming, accessed.
> (all from page 5-12 of ix484 programmer's reference)
>
> ????
>
> You need to WRITE to the text segment to be able to load executables
> (modules) and kernel threads. The user-mode code, children of `init`
> need to have writable .text segments to be able to exec().

Ah, thanks, Mr Wrong.

>
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.6.13.4 on an i686 machine (5589.48 BogoMips).
> Warning : 98.36% of all statistics are fiction.
> .
>
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> Thank you.
>
--
Coywolf Qi Hunt
http://sosdg.org/~coywolf/