Re: rwx mapping between ex_table and rodata

[Kees Cook]

On Mon, Sep 28, 2015 at 7:11 AM, Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
> On 09/24/2015 06:25 PM, Kees Cook wrote:
>> On Thu, Sep 24, 2015 at 1:26 PM, Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
>>> Hi,
>>>
>>> With the attached config and 4.3-rc2 on x86_64, I see the following in /sys/kernel/debug/kernel_page_tables:
>>> ...
>>> ---[ High Kernel Mapping ]---
>>> 0xffffffff80000000-0xffffffff81000000          16M                               pmd
>>> 0xffffffff81000000-0xffffffff81600000           6M     ro         PSE     GLB x  pmd
>>> 0xffffffff81600000-0xffffffff81775000        1492K     ro                 GLB x  pte
>>> 0xffffffff81775000-0xffffffff81800000         556K     RW                 GLB x  pte
>>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>> 0xffffffff81800000-0xffffffff81a00000           2M     ro         PSE     GLB NX pmd
>>> 0xffffffff81a00000-0xffffffff81b43000        1292K     ro                 GLB NX pte
>>> 0xffffffff81b43000-0xffffffff82000000        4852K     RW                 GLB NX pte
>>> 0xffffffff82000000-0xffffffff82200000           2M     RW         PSE     GLB NX pmd
>>> 0xffffffff82200000-0xffffffffa0000000         478M                               pmd
>>> ...
>>>
>>> This region seems to be between the end of ex_table and the start of rodata,
>>> $ objdump -x vmlinux | sort
>>> ...
>>> ffffffff817728b0 g       __ex_table     0000000000000000 __start___ex_table
>>> ffffffff817728b0 l    d  __ex_table     0000000000000000 __ex_table
>>> ffffffff81774998 g       __ex_table     0000000000000000 __stop___ex_table
>>> ffffffff81800000 g       .rodata        0000000000000000 __start_rodata
>>> ffffffff81800000 l    d  .rodata        0000000000000000 .rodata
>>> ...
>>>
>>> $ readelf -a vmlinux
>>> ...
>>> Section Headers:
>>>   [Nr] Name              Type             Address           Offset
>>>        Size              EntSize          Flags  Link  Info  Align
>>> ...
>>>   [ 3] __ex_table        PROGBITS         ffffffff817728b0  009728b0
>>>        00000000000020e8  0000000000000000   A       0     0     8
>>>   [ 4] .rodata           PROGBITS         ffffffff81800000  00a00000
>>>        00000000002eefd2  0000000000000000   A       0     0     64
>>> ...
>>>
>>> I see a similar rwx mapping with the stock Fedora kernels (e.g. 4.1.6), so it isn't new to 4.3.
>>
>> To me it looks like another alignment/padding issue like got fixed
>> before. The space between __ex_table and rodata is (seems?) unused, so
>> the default page table permissions end up being W+X. Can we fix the
>> default to be NX instead? It'll make these bugs stay gone.
>
> Not sure where that would get fixed (or the ramifications), but is there
> a reason we can't just do the following to fix this particular case?
>
> 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();
>
>

That should work, yeah. I'd still like to find the default permissions
and make them W+nx, though. Regardless, let's get the above added.

-Kees

-- 
Kees Cook
Chrome OS Security
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