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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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