linux-kernel - rwx mapping between ex

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Message-ID: <56045BC4.7000604@tycho.nsa.gov>
Date:	Thu, 24 Sep 2015 16:23:32 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	x86@...nel.og
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>
Subject: rwx mapping between ex_table and rodata

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.


View attachment "config-rwx" of type "text/plain" (160700 bytes)