Sandboxing

Linux / 19 Sep 2026 / 5 min read

seccomp User Notifications Move Selected System Calls to a Supervisor

seccomp User Notifications Move Selected System Calls to a Supervisor A seccomp filter can do more than allow a system call or reject it in the kernel. When a filter returns SECCOMP_RET_USER_NOTIF, Linux can suspend the calling thread and deliver a description of that attempted system call to a user-space supervisor. The mechanism creates an interposition boundary around selected calls. It is useful when a less-privileged process needs an operation mediated by another process, such as a container manager handling a call that the container cannot perform directly. The boundary is deliberately narrower than a general security-policy engine: notification state can race with mutable target memory, and the kernel documentation warns against treating the supervisor’s inspection as an authorization primitive.

Linux seccomp

Linux / 18 Sep 2026 / 5 min read

Seccomp User Notifications Delegate Selected System Calls to a Supervisor

A seccomp filter can stop a selected system call before execution and turn it into a request on a listener file descriptor. The calling thread remains blocked while a userspace supervisor examines the notification and returns a result. This creates a mediation boundary that is narrower than tracing every system call and more dynamic than encoding every decision directly in classic BPF. The mechanism is SECCOMP_RET_USER_NOTIF. A filter returns that action for operations that require external mediation. A filter installed with SECCOMP_FILTER_FLAG_NEW_LISTENER yields a listener file descriptor, and a supervisor uses seccomp notification ioctls on that descriptor.

Linux Seccomp

Linux / 18 Sep 2026 / 6 min read

Landlock Rulesets Add Process-Local Access Control

A Linux process can voluntarily remove access that its UID, capabilities, mount namespace, and other security layers would otherwise permit. Landlock implements this as a stackable Linux Security Module: a process creates a ruleset, adds allowed objects, then places itself in a Landlock domain. The resulting policy is an additional restriction. It does not grant access denied by DAC, ACLs, SELinux, AppArmor, mount permissions, or another active control. Once enforced, the Landlock layer cannot be removed from that thread; later Landlock domains can only add restrictions.

Linux Security

Software Engineering / 18 Sep 2026 / 4 min read

Landlock Handled Rights Define a Deny-by-Default Sandbox Boundary

A Landlock ruleset does not implicitly deny every operation known to the running kernel. It first declares which access rights it handles. Once the ruleset is enforced, those handled actions are denied by default unless a matching rule grants them. That explicit boundary is central to Landlock compatibility. User space can restrict rights it knows and has tested while a newer kernel may expose additional rights that an older binary never named.

Software Engineering Linux

Cybersecurity / 18 Sep 2026 / 7 min read

Landlock Adds a Process-Local Restriction Layer to Linux Access Control

A service may start with every filesystem permission granted to its Unix identity, yet only need a small subset after initialization. Changing the service account or mount topology can reduce that authority, but both are deployment-wide decisions. Linux Landlock provides a different boundary: a process can add restrictions to itself and its descendants without receiving privilege to grant new access. Landlock is a stackable Linux Security Module. Its rules are additional constraints, not replacements for discretionary access control, capabilities, or other active LSM policy. A Landlock rule cannot turn a denied operation into an allowed one. It can only remove authority that the process would otherwise possess.

Cybersecurity Linux

Cybersecurity / 17 Sep 2026 / 7 min read

Seccomp Filters Reduce Syscall Surface Without Forming a Complete Sandbox

Seccomp Filters Reduce Syscall Surface Without Forming a Complete Sandbox A service can run with a short seccomp allowlist and still retain broad authority through file descriptors, filesystem permissions, network endpoints, and credentials. The filter may sharply reduce the kernel interfaces reachable through system calls, yet the process can remain capable of damaging actions through operations that are explicitly allowed. This is the central boundary of seccomp: it filters syscall attempts; it does not define the full security policy of a process.

Cybersecurity Linux Security

Cybersecurity / 17 Sep 2026 / 7 min read

Landlock Rulesets Restrict Future Path Access, Not Open File Authority

Landlock Rulesets Restrict Future Path Access, Not Open File Authority A process opens a writable configuration file, installs a restrictive Landlock ruleset, and then continues running code that should have access only to a small working directory. The later policy can block a fresh attempt to open that configuration path, yet the descriptor obtained before confinement remains usable. The filesystem view has narrowed, but authority already materialized as an open file has not vanished.

Cybersecurity Linux Security