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SubjectRe: [PATCH v4 0/3] Safe, dynamically loadable LSM hooks
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On 3/6/2018 11:22 PM, Sargun Dhillon wrote:
> This patchset introduces safe dynamic LSM support. These are currently
> not unloadable, until we figure out a use case that needs that. Adding
> an unload hook is trivial given the way the patch is written.

If I recall correctly (it's been a decade since the discussion)
one of the reasons for not having loadable security modules was
that no one had a good idea on how they might be unloaded safely.
I'm not 100% convinced that there are no cases where you might
want to load a module that you can't unload, but I can definitely
come up with scenarios where it could be harmful.

> This exposes a second mechanism of loading hooks which are in modules.
> These hooks are behind static keys, so they should come at low performance
> overhead. The built-in hook heads are read-only, whereas the dynamic hooks
> are mutable.

I don't like having two mechanisms and I really don't like the
way you replace the structure of hooks with an array of pointers.

> Not all hooks can be loaded into. Some hooks are blacklisted, and therefore
> trying to load a module which plugs into those hooks will fail.

Which hooks, and why?

> One of the big benefits with loadable security modules is to help with
> "unknown unknowns". Although, livepatch is excellent, sometimes, a
> surgical LSM is simpler.

I could buy this if you could unload the module when the surgery
is done. As it is, you are stuck with whatever restrictions you
have introduced forever.

> It includes an example LSM that prevents specific time travel.
>
> Changes since v3:
> * readded hook blacklisted
> * return error, rather than panic if unable to allocate memory
>
> Changes since v2:
> * inode get/set security is readded
> * xfrm singleton hook readded
> * Security hooks are turned into an array
> * Security hooks and dynamic hooks enum is collapsed
>
> Changes since v1:
> * It no longer allows unloading of modules
> * prctl is fixed
> * inode get/set security is removed
> * xfrm singleton hook removed
>
>
> Sargun Dhillon (3):
> security: Refactor LSM hooks into an array and enum
> security: Expose a mechanism to load lsm hooks dynamically at runtime
> security: Add an example sample dynamic LSM
>
> include/linux/lsm_hooks.h | 459 ++++++++++++++++++++++++----------------------
> samples/Kconfig | 6 +
> samples/Makefile | 2 +-
> samples/lsm/Makefile | 4 +
> samples/lsm/lsm_example.c | 33 ++++
> security/Kconfig | 9 +
> security/inode.c | 13 +-
> security/security.c | 222 ++++++++++++++++++++--
> 8 files changed, 508 insertions(+), 240 deletions(-)
> create mode 100644 samples/lsm/Makefile
> create mode 100644 samples/lsm/lsm_example.c
>

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