Re: rwx mapping between ex_table and rodata

From: Kees Cook
Date: Mon Sep 28 2015 - 14:27:40 EST


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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