linux-kernel - [PATCH 0/1] Add ro_after

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Message-Id: <1465863198-15947-1-git-send-email-jeyu@redhat.com>
Date:	Mon, 13 Jun 2016 20:13:17 -0400
From:	Jessica Yu <jeyu@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>,
	Kees Cook <keescook@...gle.com>
Cc:	linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jessica Yu <jeyu@...hat.com>
Subject: [PATCH 0/1] Add ro_after_init support for modules

Hi,

This patch adds ro_after_init support for modules by adding an additional
page-aligned section in the module layout. This new ro_after_init section
sits between rodata and writable data.

So, the new module layout looks like:
   [text] [rodata] [ro_after_init] [writable data]

RO after init data remains RW during init and RO protection is enabled
separately after module init runs.

Did some light testing with lkdtm compiled as a module, verified that
ro_after_init data is writable during init, and that it oopsed after attempted
writes after init. Also tested livepatch (which uses module_{enable,disable}_ro
for its own purposes) to make sure nothing broke. More testing is appreciated :-)

Some remarks on the implementation:
 * A new SHF_RO_AFTER_INIT flag is introduced in elf.h to make
   identification of .data..ro_after_init sections and the work of
   layout_sections() easier. Its chosen value is within the SHF_MASKOS
   range. If people don't like adding a new SHF flag to elf.h, I could
   just make the flag internal to module.c.

 * frob_ro_after_init() could have been separated from
   module_enable_ro() (i.e., put it in its own function, something
   like module_enable_ro_after_init()), but given that livepatch also
   uses module_enable_ro(), I did not want to make livepatch worry
   about calling yet another function just to re-enable all RO protections
   for a module.

 * If a module doesn't have a ro_after_init section, then
   core_layout.ro_after_init_size just takes the value of
   core_layout.ro_size, and frob_ro_after_init() should do nothing.

Based on linux-next.

Previous discussion here:
http://comments.gmane.org/gmane.linux.kernel/2234606

Jessica Yu (1):
  modules: add ro_after_init support

 include/linux/module.h   |  2 ++
 include/uapi/linux/elf.h |  1 +
 kernel/module.c          | 73 +++++++++++++++++++++++++++++++++++++++++-------
 3 files changed, 66 insertions(+), 10 deletions(-)

-- 
2.4.3