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SubjectRe: [patch] mark text section read-only
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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().

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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