• Alexei Starovoitov's avatar
    umh: introduce fork_usermode_blob() helper · 449325b5
    Alexei Starovoitov authored
    Introduce helper:
    int fork_usermode_blob(void *data, size_t len, struct umh_info *info);
    struct umh_info {
           struct file *pipe_to_umh;
           struct file *pipe_from_umh;
           pid_t pid;
    that GPLed kernel modules (signed or unsigned) can use it to execute part
    of its own data as swappable user mode process.
    The kernel will do:
    - allocate a unique file in tmpfs
    - populate that file with [data, data + len] bytes
    - user-mode-helper code will do_execve that file and, before the process
      starts, the kernel will create two unix pipes for bidirectional
      communication between kernel module and umh
    - close tmpfs file, effectively deleting it
    - the fork_usermode_blob will return zero on success and populate
      'struct umh_info' with two unix pipes and the pid of the user process
    As the first step in the development of the bpfilter project
    the fork_usermode_blob() helper is introduced to allow user mode code
    to be invoked from a kernel module. The idea is that user mode code plus
    normal kernel module code are built as part of the kernel build
    and installed as traditional kernel module into distro specified location,
    such that from a distribution point of view, there is
    no difference between regular kernel modules and kernel modules + umh code.
    Such modules can be signed, modprobed, rmmod, etc. The use of this new helper
    by a kernel module doesn't make it any special from kernel and user space
    tooling point of view.
    Such approach enables kernel to delegate functionality traditionally done
    by the kernel modules into the user space processes (either root or !root) and
    reduces security attack surface of the new code. The buggy umh code would crash
    the user process, but not the kernel. Another advantage is that umh code
    of the kernel module can be debugged and tested out of user space
    (e.g. opening the possibility to run clang sanitizers, fuzzers or
    user space test suites on the umh code).
    In case of the bpfilter project such architecture allows complex control plane
    to be done in the user space while bpf based data plane stays in the kernel.
    Since umh can crash, can be oom-ed by the kernel, killed by the admin,
    the kernel module that uses them (like bpfilter) needs to manage life
    time of umh on its own via two unix pipes and the pid of umh.
    The exit code of such kernel module should kill the umh it started,
    so that rmmod of the kernel module will cleanup the corresponding umh.
    Just like if the kernel module does kmalloc() it should kfree() it
    in the exit code.
    Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
umh.c 18 KB