lkml.org 
[lkml]   [2015]   [Sep]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: rwx mapping between ex_table and rodata
From
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.

-Kees

--
Kees Cook
Chrome OS Security


\
 
 \ /
  Last update: 2015-09-25 00:41    [W:0.126 / U:0.168 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site