sys_linux: add support for seccomp filters

The Linux secure computing (seccomp) facility allows a process to
install a filter in the kernel that will allow only specific system
calls to be made. The process is killed when trying to make other system
calls. This is useful to reduce the kernel attack surface and possibly
prevent kernel exploits when the process is compromised.

Use the libseccomp library to add rules and load the filter into the
kernel. Keep a list of system calls that are always allowed after
chronyd is initialized. Restrict arguments that may be passed to the
socket(), setsockopt(), fcntl(), and ioctl() system calls. Arguments
to socketcall(), which is used on some architectures as a multiplexer
instead of separate socket system calls, are not restricted for now.
The mailonchange directive is not allowed as it calls sendmail.

Calls made by the libraries that chronyd is using have to be covered
too. It's difficult to determine which system calls they need as it may
change after an upgrade and it may depend on their configuration (e.g.
resolver in libc). There are also differences between architectures. It
can all break very easily and is therefore disabled by default. It can
be enabled with the new -F option.

This is based on a patch from Andrew Griffiths <agriffit@redhat.com>.

sys.h

@@ -38,6 +38,10 @@ extern void SYS_Finalise(void);
 /* Drop root privileges to the specified user and group */
 extern void SYS_DropRoot(uid_t uid, gid_t gid);
 
+/* Enable a system call filter to allow only system calls
+   which chronyd normally needs after initialization */
+extern void SYS_EnableSystemCallFilter(int level);
+
 extern void SYS_SetScheduler(int SchedPriority);
 extern void SYS_LockMemory(void);