Re: rwx mapping between ex_table and rodata

[Kees Cook]

On Fri, Sep 25, 2015 at 12:22 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Kees Cook <keescook@xxxxxxxxxxxx> 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.
>
> Yeah. Wanna send a patch for that?

I haven't found where that is actually happening. :( If anyone has
pointers, I can dig a bit more.

-Kees

-- 
Kees Cook
Chrome OS Security
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