[l0kod]

BSD systems ship a kernel and user space, which simplifies a lot of things. Linux is more flexible but it comes at a cost. Adding new security features can also be challenging for other reasons. Anyway, Landlock is one of these new security primitives, and it is gaining new features over time.

The Landlock interface must not change the underlying semantic of what is allowed or denied, otherwise it could break apps build for an older or a newer kernel. However, these apps should still use all the available security features. This is challenging.

Landlock provides a way to define fine-grained security policies. I would not say the kernel interface is complex (rather flexible), but what really matter are the user space library interfaces and how they can safely abstract complexity.

[__turbobrew__]

I know how linux and bsd work. I still have yet to find a satisfactory answer to why linux cannot create security primitives which are useful — like wireguard. I understand that landlock tries to abstract complexity, but why do we need to design complex user interfaces? Pledge and unveil are just simple syscalls, there is no magic secret sauce on BSDs which enable these syscalls. It is true that bsd userspace has been compiled to bake in plege and unviel syscalls, but that is totally separate from the usability of the interfaces.

[l0kod]

For instance, with Pledge, the "dns" promise is implemented with hardcoded path in the kernel. Linux is complex because it is versatile and flexible. Controlling access to such features requires some complexity and the kernel might not be enough.

About interfaces, another example is that Unveil is configured with path names but Landlock uses file descriptors instead (more flexible).

Also, these OpenBSD primitives only apply to the current executed binary, there is no nested sandboxes because the goal is not to create this kind of secure environment but mainly to secure a trusted binary.

[mkj]

For a given linux libc function (what a program calls), the underlying kernel syscall might change over time or vary for other reasons. Since the landlock/seccomp filters are at the kernel level, that breaks programs which only interact with libc calls and don't expect different behaviour.

[yellowapple]

The underlying kernel syscall should never change, though, right? Pretty sure that's the sort of userspace-backwards-compatibility-breaking change that would result in one of Linus' famous angry emails.

[mkj]

Things like clock_gettime64() to handle dates past 2038.

Calling clock_gettime() in libc will call the newer syscall (assuming __TIME_BITS=64 is set). But Linux has kept backwards compat, old programs can still call the old syscall.

If you wrote your seccomp rule for your program before clock_gettime64 existed, it'd break when glibc switched. I guess that implies each language stdlib should have their own seccomp etc wrappers.

For landlock, the equivalent is that glibc reads various files in /etc varying per libc version or system settings, so landlock rules need to account for that.

[__turbobrew__]

> Since the landlock/seccomp filters are at the kernel level

That arguably shows that seccomp is operating at the wrong abstraction level, or the kernel needs another higher level api. With pledge, you operate on capabilities and as new functionality is added to the kernel it is categorized under existing capabilities (for example, if your program pledges not to use networking you can assume that it should not be able to use new networking syscalls added to the kernel in the future).

[l0kod]

Seccomp is not an access control system, but Landlock is. Seccomp limits the kernel attack surface and Landlock enforces an access control. They are complementary.

With Landlock, the access control is at the right layer, and the semantic is guaranteed to be the same even if the kernel gets new syscalls. Landlock is the closest thing to Pledge/Unveil we can get with the Linux constraints (and it is gaining new features).