Re: rwx mapping between ex_table and rodata

[Stephen Smalley <sds@tycho.nsa.gov>]

On 09/24/2015 06:25 PM, Kees Cook wrote:
> On Thu, Sep 24, 2015 at 1:26 PM, Stephen Smalley <sds@tycho.nsa.gov> 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();