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SubjectRe: rwx mapping between ex_table and rodata

* 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


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