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  1. May 24, 2018
    • David S. Miller's avatar
      Merge branch 'bpfilter' · e95a5f54
      David S. Miller authored 
      Alexei Starovoitov says:

====================
bpfilter

v2->v3:
- followed Luis's suggestion and significantly simplied first patch
  with shmem_kernel_file_setup+kernel_write. Added kdoc for new helper
- fixed typos and race to access pipes with mutex
- tested with bpfilter being 'builtin'. CONFIG_BPFILTER_UMH=y|m both work.
  Interesting to see a usermode executable being embedded inside vmlinux.
- it doesn't hurt to enable bpfilter in .config.
  ip_setsockopt commands sent to usermode via pipes and -ENOPROTOOPT is
  returned from userspace, so kernel falls back to original iptables code

v1->v2:
this patch set is almost a full rewrite of the earlier umh modules approach
The v1 of patches and follow up discussion was covered by LWN:
https://lwn.net/Articles/749108/



I believe the v2 addresses all issues brought up by Andy and others.
Mainly there are zero changes to kernel/module.c
Instead of teaching module loading logic to recognize special
umh module, let normal kernel modules execute part of its own
.init.rodata as a new user space process (Andy's idea)
Patch 1 introduces this new helper:
int fork_usermode_blob(void *data, size_t len, struct umh_info *info);
Input:
  data + len == executable file
Output:
  struct umh_info {
       struct file *pipe_to_umh;
       struct file *pipe_from_umh;
       pid_t pid;
  };

Advantages vs v1:
- the embedded user mode executable is stored as .init.rodata inside
  normal kernel module. These pages are freed when .ko finishes loading
- the elf file is copied into tmpfs file. The user mode process is swappable.
- the communication between user mode process and 'parent' kernel module
  is done via two unix pipes, hence protocol is not exposed to
  user space
- impossible to launch umh on its own (that was the main issue of v1)
  and impossible to be man-in-the-middle due to pipes
- bpfilter.ko consists of tiny kernel part that passes the data
  between kernel and umh via pipes and much bigger umh part that
  doing all the work
- 'lsmod' shows bpfilter.ko as usual.
  'rmmod bpfilter' removes kernel module and kills corresponding umh
- signed bpfilter.ko covers the whole image including umh code

Few issues:
- the user can still attach to the process and debug it with
  'gdb /proc/pid/exe pid', but 'gdb -p pid' doesn't work.
  (a bit worse comparing to v1)
- tinyconfig will notice a small increase in .text
  +766 | TEXT | 7c8b94806bec umh: introduce fork_usermode_blob() helper
====================

Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      e95a5f54
    • Alexei Starovoitov's avatar
      net: add skeleton of bpfilter kernel module · d2ba09c1
      Alexei Starovoitov authored 
      

bpfilter.ko consists of bpfilter_kern.c (normal kernel module code)
and user mode helper code that is embedded into bpfilter.ko

The steps to build bpfilter.ko are the following:
- main.c is compiled by HOSTCC into the bpfilter_umh elf executable file
- with quite a bit of objcopy and Makefile magic the bpfilter_umh elf file
  is converted into bpfilter_umh.o object file
  with _binary_net_bpfilter_bpfilter_umh_start and _end symbols
  Example:
  $ nm ./bld_x64/net/bpfilter/bpfilter_umh.o
  0000000000004cf8 T _binary_net_bpfilter_bpfilter_umh_end
  0000000000004cf8 A _binary_net_bpfilter_bpfilter_umh_size
  0000000000000000 T _binary_net_bpfilter_bpfilter_umh_start
- bpfilter_umh.o and bpfilter_kern.o are linked together into bpfilter.ko

bpfilter_kern.c is a normal kernel module code that calls
the fork_usermode_blob() helper to execute part of its own data
as a user mode process.

Notice that _binary_net_bpfilter_bpfilter_umh_start - end
is placed into .init.rodata section, so it's freed as soon as __init
function of bpfilter.ko is finished.
As part of __init the bpfilter.ko does first request/reply action
via two unix pipe provided by fork_usermode_blob() helper to
make sure that umh is healthy. If not it will kill it via pid.

Later bpfilter_process_sockopt() will be called from bpfilter hooks
in get/setsockopt() to pass iptable commands into umh via bpfilter.ko

If admin does 'rmmod bpfilter' the __exit code bpfilter.ko will
kill umh as well.

Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
      d2ba09c1
    • 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>
      449325b5
  3. May 23, 2018
  5. May 22, 2018