Re: rwx mapping between ex_table and rodata

[Kees Cook]

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.

-Kees

-- 
Kees Cook
Chrome OS Security
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