isolation of libmagic within Python

magic.py

import os
import socket
import sys
import Pyro5.api
import Pyro5.server
import pyseccomp as seccomp
import serpent


@Pyro5.server.expose
class MagicServer:
    def from_buffer(self, data):
        from magic import from_buffer

        data = serpent.tobytes(data)
        return from_buffer(data)


def pyro5_server(sock):
    with Pyro5.api.Daemon(connected_socket=sock) as daemon:
        daemon.register(MagicServer, objectId="magic.server")
        daemon.requestLoop()


def drop_privileges(sock):
    # I would also use namespaces, but most of the interesting ones require
    # CAP_SYS_ADMIN
    sc_filter = seccomp.SyscallFilter(seccomp.ERRNO(seccomp.errno.EPERM))
    fileno = sock.fileno()

    sc_filter.add_rule(
        seccomp.ALLOW, "sendto", seccomp.Arg(0, seccomp.EQ, fileno)
    )

    sc_filter.add_rule(
        seccomp.ALLOW, "recvfrom", seccomp.Arg(0, seccomp.EQ, fileno)
    )
    sc_filter.add_rule(seccomp.ALLOW, "read")
    sc_filter.add_rule(seccomp.ALLOW, "write")
    sc_filter.add_rule(seccomp.ALLOW, "close")
    sc_filter.add_rule(seccomp.ALLOW, "lseek")
    sc_filter.add_rule(seccomp.ALLOW, "getpeername")
    sc_filter.add_rule(seccomp.ALLOW, "getsockname")
    sc_filter.add_rule(seccomp.ALLOW, "getrandom")
    sc_filter.add_rule(seccomp.ALLOW, "mmap")
    sc_filter.add_rule(seccomp.ALLOW, "munmap")
    sc_filter.add_rule(seccomp.ALLOW, "shutdown")
    sc_filter.add_rule(seccomp.ALLOW, "mprotect")
    sc_filter.add_rule(seccomp.ALLOW, "uname")

    # A bpf filter to restrict to sys.path would be nice
    sc_filter.add_rule(seccomp.ALLOW, "getdents64")
    sc_filter.add_rule(seccomp.ALLOW, "newfstatat")

    allowed_flags = os.O_RDONLY | os.O_NONBLOCK | os.O_CLOEXEC | os.O_DIRECTORY
    disallowed_flags = (1 << 32) - 1 - allowed_flags
    sc_filter.add_rule(
        seccomp.ALLOW,
        "openat",
        seccomp.Arg(0, seccomp.EQ, 4294967196),
        seccomp.Arg(2, seccomp.MASKED_EQ, disallowed_flags, 0),
    )

    sc_filter.load()


def parent_process(sock):
    buf = open(sys.argv[1], "rb").read(2048)
    with Pyro5.api.Proxy("magic.server", connected_socket=sock) as magic_proxy:
        result = magic_proxy.from_buffer(buf)
        print("Magic type:", result)


def main():
    parent_sock, child_sock = socket.socketpair()
    pid = os.fork()

    if pid == 0:
        parent_sock.close()
        drop_privileges(child_sock)
        pyro5_server(child_sock)
    else:
        child_sock.close()
        parent_process(parent_sock)


if __name__ == "__main__":
    raise SystemExit(main())