[PATCH 2/3] lsm: introduce security_lsm_manage_policy hook

[Mickaël Salaün]

On Thu, May 08, 2025 at 09:54:19AM -0700, Casey Schaufler wrote:
> On 5/8/2025 1:29 AM, John Johansen wrote:
> > On 5/7/25 13:25, Paul Moore wrote:
> >> On Wed, May 7, 2025 at 6:41 AM Tetsuo Handa
> >> <penguin-kernel at i-love.sakura.ne.jp> wrote:
> >>> On 2025/05/06 23:32, Maxime Bélair wrote:
> >>>> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
> >>>> index dcaad8818679..b39e6635a7d5 100644
> >>>> --- a/security/lsm_syscalls.c
> >>>> +++ b/security/lsm_syscalls.c
> >>>> @@ -122,5 +122,10 @@ SYSCALL_DEFINE3(lsm_list_modules, u64 __user
> >>>> *, ids, u32 __user *, size,
> >>>>   SYSCALL_DEFINE5(lsm_manage_policy, u32, lsm_id, u32, op, void
> >>>> __user *, buf, u32
> >>>>                __user *, size, u32, flags)
> >>>>   {
> >>>> -     return 0;
> >>>> +     size_t usize;
> >>>> +
> >>>> +     if (get_user(usize, size))
> >>>> +             return -EFAULT;
> >>>> +
> >>>> +     return security_lsm_manage_policy(lsm_id, op, buf, usize,
> >>>> flags);
> >>>>   }
> >>>
> >>> syzbot will report user-controlled unbounded huge size memory
> >>> allocation attempt. ;-)
> >>>
> >>> This interface might be fine for AppArmor, but TOMOYO won't use this
> >>> interface because
> >>> TOMOYO's policy is line-oriented ASCII text data where the
> >>> destination is switched via
> >>> pseudo‑filesystem's filename ...
> >>
> >> While Tetsuo's comment is limited to TOMOYO, I believe the argument
> >> applies to a number of other LSMs as well.  The reality is that there
> >> is no one policy ideal shared across LSMs and that complicates things
> >> like the lsm_manage_policy() proposal.  I'm intentionally saying
> >> "complicates" and not "prevents" because I don't want to flat out
> >> reject something like this, but I think there needs to be a larger
> >> discussion among the different LSM groups about what such an API
> >> should look like.  We may not need to get every LSM to support this
> >> new API, but we need to get something that would work for a
> >> significant majority and would be general/extensible enough that we
> >> would expect it to work with the majority of future LSMs (as much as
> >> we can predict the future anyway).
> >>
> >
> > yep, I look at this is just a starting point for discussion. There
> > isn't going to be any discussion without some code, so here is a v1
> > that supports a single LSM let the bike shedding begin.
> 
> Aside from the issues with allocating a buffer for a big policy
> I don't see a problem with this proposal. The system call looks
> a lot like the other LSM interfaces, so any developer who likes
> those ought to like this one. The infrastructure can easily check
> the lsm_id and only call the appropriate LSM hook, so no one
> is going to be interfering with other modules.

We may not want to only be able to load buffers containing policies, but
also to leverage file descriptors like Landlock does.  Getting a
property from a kernel object or updating it is mainly about dealing
with a buffer.  And the current LSM syscalls do just that.  Other kind
of operations may require more than that though.

I don't like multiplexer syscalls because they don't expose a clear
semantic and can be complex to manage and filter.  This new syscall is
kind of a multiplexer that redirect commands to an arbitrary set of
kernel parts, which can then define their own semantic.  I'd like to see
a clear set of well-defined operations and their required permission.
Even better, one syscall per operation should simplify their interface.