docs: update seccomp documentation

Replace outdated TODO comment with updated information about runc's seccomp support.

Signed-off-by: Osama Abdelkader <[email protected]>

Author: osamakader

libcontainer/SPEC.md

@@ -367,7 +367,26 @@ profile <profile_name> flags=(attach_disconnected,mediate_deleted) {
 }
 ```
 
-*TODO: seccomp work is being done to find a good default config*
+[seccomp](https://en.wikipedia.org/wiki/Seccomp) can be used to apply filters
+to the system calls used in a container. The set of filter expressions allows
+you to match against syscall numbers (automatically resolved from syscall
+names) and apply various comparison operators to syscall arguments.
+
+When a filter rule matches, the associated action will occur (this could
+involve killing the process or thread, returning an errno value without
+executing the syscall, forwarding the request to a user-space agent to handle,
+emitting a log entry, or permitting the syscall to execute).
+
+The primary use-case is to provide an explicit allow-list of syscalls for a
+container, to reduce the kernel API attack surface exposed the container.
+Historically, seccomp has protected containers against various kernel 0-day
+vulnerabilities, so a strong seccomp filter is highly recommended.
+
+libcontainer does not currently provide a default filter, but higher-level
+runtimes tend to define their own filters for use with runc. Here is an example
+OCI seccomp profile (see
+[oci-runtime-seccomp](https://github.com/opencontainers/runtime-spec/blob/v1.2.1/config-linux.md#seccomp)
+for more information on how to write your own filters)
 
 ### Runtime and Init Process