Re: [PATCH] x86/mm: Set NX on gap between __ex_table and rodata

[Kees Cook]

On Thu, Oct 1, 2015 at 6:04 AM, Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
> Unused space between the end of __ex_table and the start of rodata
> can be left W+x in the kernel page tables.  Extend the setting
> of the NX bit to cover this gap by starting from text_end rather than
> rodata_start.
>
> Before:
> ---[ High Kernel Mapping ]---
> 0xffffffff80000000-0xffffffff81000000          16M                               pmd
> 0xffffffff81000000-0xffffffff81600000           6M     ro         PSE     GLB x  pmd
> 0xffffffff81600000-0xffffffff81754000        1360K     ro                 GLB x  pte
> 0xffffffff81754000-0xffffffff81800000         688K     RW                 GLB x  pte
> 0xffffffff81800000-0xffffffff81a00000           2M     ro         PSE     GLB NX pmd
> 0xffffffff81a00000-0xffffffff81b3b000        1260K     ro                 GLB NX pte
> 0xffffffff81b3b000-0xffffffff82000000        4884K     RW                 GLB NX pte
> 0xffffffff82000000-0xffffffff82200000           2M     RW         PSE     GLB NX pmd
> 0xffffffff82200000-0xffffffffa0000000         478M                               pmd
>
> After:
> ---[ High Kernel Mapping ]---
> 0xffffffff80000000-0xffffffff81000000          16M                               pmd
> 0xffffffff81000000-0xffffffff81600000           6M     ro         PSE     GLB x  pmd
> 0xffffffff81600000-0xffffffff81754000        1360K     ro                 GLB x  pte
> 0xffffffff81754000-0xffffffff81800000         688K     RW                 GLB NX pte
> 0xffffffff81800000-0xffffffff81a00000           2M     ro         PSE     GLB NX pmd
> 0xffffffff81a00000-0xffffffff81b3b000        1260K     ro                 GLB NX pte
> 0xffffffff81b3b000-0xffffffff82000000        4884K     RW                 GLB NX pte
> 0xffffffff82000000-0xffffffff82200000           2M     RW         PSE     GLB NX pmd
> 0xffffffff82200000-0xffffffffa0000000         478M                               pmd
>
> Signed-off-by: Stephen Smalley <sds@xxxxxxxxxxxxx>

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/x86/mm/init_64.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 30564e2..df48430 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -1132,7 +1132,7 @@ void mark_rodata_ro(void)
>          * has been zapped already via cleanup_highmem().
>          */
>         all_end = roundup((unsigned long)_brk_end, PMD_SIZE);
> -       set_memory_nx(rodata_start, (all_end - rodata_start) >> PAGE_SHIFT);
> +       set_memory_nx(text_end, (all_end - text_end) >> PAGE_SHIFT);
>
>         rodata_test();
>
> --
> 2.1.0
>



-- 
Kees Cook
Chrome OS Security
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