Re: rwx mapping between ex_table and rodata

[Ingo Molnar <mingo@kernel.org>]

* Kees Cook <keescook@chromium.org> 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.

Yeah. Wanna send a patch for that?

Thanks,

	Ingo