Re: [PATCH 1/3] Wire up the lsm_manage_policy syscall
From: John Johansen
Date: Sun May 11 2025 - 06:47:46 EST
On 5/9/25 03:26, Mickaël Salaün wrote:
On Thu, May 08, 2025 at 01:18:20AM -0700, John Johansen wrote:
On 5/7/25 23:06, Song Liu wrote:
On Wed, May 7, 2025 at 8:37 AM Maxime Bélair
<maxime.belair@xxxxxxxxxxxxx> wrote:
[...]
These two do not feel like real benefits:
- One syscall cannot fit all use cases well...
This syscall is not intended to cover every case, nor to replace existing kernel
interfaces.
Each LSM can decide which operations it wants to support (if any). For example, when
loading policies, an LSM may choose to allow only policies that further restrict
privileges.
- Not working in containers is often not an issue, but a feature.
Indeed, using this syscall requires appropriate capabilities and will not permit
unprivileged containers to manage policies arbitrarily.
With this syscall, capability checks remain the responsibility of each LSM.
For instance, in the AppArmor patch, a profile can be loaded only if
aa_policy_admin_capable() succeeds (which requires CAP_MAC_ADMIN). Moreover, by design,
policies can be loaded only in the current namespace.
I see this syscall as a middle point between exposing the entire sysfs, creating a large
attack surface, and blocking everything.
Landlock’s existing syscalls already improve security by allowing processes to further
restrict their ambient rights while adding only a modest attack surface.
This syscall is a further step in that direction: it lets LSMs add restrictive policies
without requiring exposing every other interface.
I don't think a syscall makes the API more secure. If necessary, we can add
It exposes a different attack surface. Requiring mounting of the fs to where it is visible
in the container, provides attack surface, and requires additional external configuration.
We should also keep in mind that syscalls could be accessible from
everywhere, by everyone, which may increase the attack surface compared
to a privileged filesystem interface. Adding a second interface may
also introduce issues. Anyway, I'm definitely not against syscalls, but
I don't see why the filesystem interface would be "less secure" in this
context.
yes syscalls being accessible from everywhere is another form of attack
surface, that needs to be mediated.
the fs can be mediated, its expose is a multiple lsms with multiple
different interfaces on the files within it. What really is more
problematic is makng the fs available in the container. Yes a
container manager can do it but then you are dependent on the
container manager making your interface available.
Other wise you are looking at making mount available to your app
within the container.
Then there is the whole issue of getting the various LSMs to allow another LSM in the
stack to be able manage its own policy.
Right, and it's a similar issue with seccomp policies wrt syscalls.
yes, though seccomp I have found to be the easier one to deal with
permission check to each pseudo file. The downside of the syscall, however,
is that all the permission checks are hard-coded in the kernel (except for
The permission checks don't have to be hard coded. Each LSM can define how it handles
or manages the syscall. The default is that it isn't supported, but if an lsm decides
to support it, there is now reason that its policy can't determine the use of the
syscall.
From an interface design point of view, it would be better to clearly
specify the scope of a command (e.g. which components could be impacted
by a command), and make sure the documentation reflect that as well.
Even better, have a syscalls per required privileges and impact (e.g.
privileged or unprivileged). Going this road, I'm not sure if a
privileged syscall would make sense given the existing filesystem
interface.
uhhhmmm, not just privileged. As you well know we are looking to use
this for unprivileged policy. The LSM can limit to privileged if it
wants but it doesn't have to limit it to privileged policy.
BPF LSM); while the sys admin can configure permissions of the pseudo
files in user space.
Other LSMs also have policy that can control access to pseudo filesystems and
other resources. Again, the control doesn't have to be hard coded. And seccomp can
be used to block the syscall.
Again, each module decides which operations to expose through this syscall. In many cases
the operation will still require CAP_SYS_ADMIN or a similar capability, so environments
that choose this interface remain secure while gaining its advantages.
- Avoids overhead of other kernel interfaces for better efficiency
.. and it is is probably less efficient, because everything need to
fit in the same API.
As shown below, the syscall can significantly improve the performance of policy management.
A more detailed benchmark is available in [1].
The following table presents the time required to load an AppArmor profile.
For every cell, the first value is the total time taken by aa-load, and the value in
parentheses is the time spent to load the policy in the kernel only (total - dry‑run).
Results are in microseconds and are averaged over 10 000 runs to reduce variance.
| t (µs) | syscall | pseudofs | Speedup |
|-----------|-------------|-------------|---------------|
| 1password | 4257 (1127) | 3333 (192) | x1.28 (x5.86) |
| Xorg | 6099 (2961) | 5167 (2020) | x1.18 (x1.47) |
I am not sure the performance of loading security policies is on any
critical path.
generally speaking I agree, but I am also not going to turn down a
performance improvement either. Its a nice to have, but not a strong
argument for need.
The implementation calls the hook for each LSM, which is why I think the
syscall is not efficient.
it should only call the LSM identified by the lsmid in the call.
Overall, I am still not convinced a syscall for all LSMs is needed. To
justify such
its not needed by all LSMs, just a subset of them, and some nebulous
subset of potentially future LSMs that is entirely undefinable.
If we had had appropriate LSM syscalls landlock wouldn't have needed
to have landlock specific syscalls. Having another LSM go that route
feels wrong especially now that we have some LSM syscalls.
I don't agree. Dedicated syscalls are a good thing. See my other
reply.
I think we can just disagree on this point.
If a
syscall is needed by an LSM its better to try hashing something out
that might have utility for multiple LSMs or at the very least,
potentially have utility in the future.
a syscall, I think we need to show that it is useful in multiple LSMs.
Also, if we
really want to have single set of APIs for all LSMs, we may also need
get_policy,
We are never going to get a single set of APIs for all LSMs. I will
settle for an api that has utility for a subset
remove_policy, etc. This set as-is appears to be an incomplete design. The
To have a complete design, there needs to be feedback and discussion
from multiple LSMs. This is a starting point.
implementation, with call_int_hook, is also problematic. It can easily
cause some> controversial behaviors.
agreed it shouldn't be doing a straight call_int_hook, it should only
call it against the lsm identified by the lsmid
Yes, but then, I don't see the point of a "generic" LSM syscall.
its not a generic LSM syscall. Its a syscall or maybe a set of syscalls
for a specific scoped problem of loading/managing policy.
Can we come to something acceptable? I don't know but we are going to
look at it before trying for an apparmor specific syscall.