Re: [kernel-hardening] [PATCH 0/7] introduce __ro_mostly_after_init section marker
From: Ho-Eun Ryu
Date: Tue Feb 21 2017 - 01:22:43 EST
> On 19 Feb 2017, at 8:14 PM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> Hi Hoeun,
> On 19 February 2017 at 10:03, Hoeun Ryu <hoeun.ryu@xxxxxxxxx> wrote:
>> After `__ro_after_init` marker is included in kernel, many kernel data
>> objects can be read-only-after-init. But there are many other places that
>> would be good to read-only-after-init but `__ro_after_init` can not be simply
>> applicable to them because they should be writable at some points, which are
>> during module_init/exit or dynamic de/registration for a specific subsystem.
> The argument sounds reasonable, but it would really help if you could
> describe some use cases in more detail.
Please see RFC 4/7  and RFC 5/7 .
RFC 4 shows that cpu_ap/bp_states are marked as __ro_mostly_after_init.
cpu_ap/bp_states can not be marked as __ro_after_init because some modules
can write to those objects using cpuhp_setup/remove_state api during module_init/exit.
RFC 5 shows that selinux_hooks and selinux_nf_hooks are marked as __ro_mostly_after_init.
those hooks can not be marked as __ro_after_init because selinux_disable() function.
Those hooks should be writable during selinux_disable to deregister the hooks.
>> `__ro_mostly_after_init` is basically the same to `__ro_after_init`. The
>> section is mapped as read-only after kernel init. The different thing is
>> this section is temporarily mapped as read-write during module_init/exit and
>> de/registration of a subsystem using set_ro_mostly_after_init_rw/ro pair.
>> - Tested only on arm64.
>> 0001 patch is `__ro_mostly_after_init` itself.
>> 0002 patch is to add set_ro_mostly_after_init_rw/ro pair using
>> 0003 patch is to make the section read-write in module_init/exit.
>> 0004 patch is an example for dynamic init/deinit of a subsystem.
>> 0005 patch is an example for __ro_mostly_after_init section modified during
>> 0006/0007 patches are fixes for arm64 kernel mapping.
>> Hoeun Ryu (7):
>> arch: add __ro_mostly_after_init section marker
>> init: add set_ro_mostly_after_init_rw/ro function
>> module: modify memory attrs for __ro_mostly_after_init during
>> selinux: mark __ro_mostly_after_init for selinux_hooks/selinux_nf_ops
>> cpu: mark ro_mostly_after_init for cpuhp_ap/bp_states
>> arm64: add __map_kernel_segment to accept additional vm flags
>> arm64: map seperately rodata sections for __ro_mostly_after_init
>> arch/arm64/mm/mmu.c | 44 ++++++++++++++++++++++++++++++++-------
>> include/asm-generic/sections.h | 1 +
>> include/asm-generic/vmlinux.lds.h | 10 +++++++++
>> include/linux/cache.h | 11 ++++++++++
>> include/linux/init.h | 6 ++++++
>> init/main.c | 24 +++++++++++++++++++++
>> kernel/cpu.c | 4 ++--
>> kernel/module.c | 10 +++++++--
>> security/selinux/hooks.c | 8 +++++--
>> 9 files changed, 105 insertions(+), 13 deletions(-)