Re: [PATCH v7 0/7] Fork brute force attack mitigation

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Moreover, I think this solves another problem pointed out by Andi Kleen
during the v5 review [2] related to the possibility that a supervisor
respawns processes killed by the Brute LSM. He suggested adding some way so
a supervisor can know that a process has been killed by Brute and then
decide to respawn or not. So, now, the supervisor can read the brute xattr
of one executable and know if it is blocked by Brute and why (using the
statistical data).

It looks better now, Thank.

One potential problem is that the supervisor might see the executable directly, but run it through some wrapper. In fact I suspect that will be fairly common with complex daemons. So it couldn't directly look at the xattr. Might be useful to also pass this information through the wait* chain, so that the supervisor can directly collect it. That would need some extension to these system calls.



Knowing all this information I will explain now the different patches:

The 1/7 patch defines a new LSM hook to get the fatal signal of a task.
This will be useful during the attack detection phase.

The 2/7 patch defines a new LSM and the necessary sysctl attributes to fine
tuning the attack detection.

The 3/7 patch detects a fork/exec brute force attack and narrows the
possible cases taken into account the privilege boundary crossing.

The 4/7 patch mitigates a brute force attack.

The 5/7 patch adds self-tests to validate the Brute LSM expectations.

The 6/7 patch adds the documentation to explain this implementation.

The 7/7 patch updates the maintainers file.

This patch serie is a task of the KSPP [3] and can also be accessed from my
github tree [4] in the "brute_v7" branch.


When I ran the "checkpatch" script I got the following errors, but I think
they are false positives as I follow the same coding style for the others
extended attributes suffixes.

ERROR: Macros with complex values should be enclosed in parentheses
89: FILE: include/uapi/linux/xattr.h:80:

ERROR: Macros with complex values should be enclosed in parentheses
100: FILE: tools/testing/selftests/brute/rmxattr.c:18:

When I ran the "kernel-doc" script with the following parameters:

./scripts/kernel-doc --none -v security/brute/brute.c

I got the following warning:

security/brute/brute.c:65: warning: contents before sections

But I don't understand why it is complaining. Could it be a false positive?

The previous versions can be found in:


Version 2

Version 3

Version 4

Version 5

Version 6

Changelog RFC -> v2
- Rename this feature with a more suitable name (Jann Horn, Kees Cook).
- Convert the code to an LSM (Kees Cook).
- Add locking  to avoid data races (Jann Horn).
- Add a new LSM hook to get the fatal signal of a task (Jann Horn, Kees
- Add the last crashes timestamps list to avoid false positives in the
   attack detection (Jann Horn).
- Use "period" instead of "rate" (Jann Horn).
- Other minor changes suggested (Jann Horn, Kees Cook).

Changelog v2 -> v3
- Compute the application crash period on an on-going basis (Kees Cook).
- Detect a brute force attack through the execve system call (Kees Cook).
- Detect an slow brute force attack (Randy Dunlap).
- Fine tuning the detection taken into account privilege boundary crossing
   (Kees Cook).
- Taken into account only fatal signals delivered by the kernel (Kees
- Remove the sysctl attributes to fine tuning the detection (Kees Cook).
- Remove the prctls to allow per process enabling/disabling (Kees Cook).
- Improve the documentation (Kees Cook).
- Fix some typos in the documentation (Randy Dunlap).
- Add self-test to validate the expectations (Kees Cook).

Changelog v3 -> v4
- Fix all the warnings shown by the tool "scripts/kernel-doc" (Randy

Changelog v4 -> v5
- Fix some typos (Randy Dunlap).

Changelog v5 -> v6
- Fix a reported deadlock (kernel test robot).
- Add high level details to the documentation (Andi Kleen).

Changelog v6 -> v7

- Add the "Reviewed-by:" tag to the first patch.
- Rearrange the brute LSM between lockdown and yama (Kees Cook).
- Split subdir and obj in security/Makefile (Kees Cook).
- Reduce the number of header files included (Kees Cook).
- Print the pid when an attack is detected (Kees Cook).
- Use the socket_accept LSM hook instead of socket_sock_rcv_skb hook to
   avoid running a hook on every incoming network packet (Kees Cook).
- Update the documentation and fix it to render it properly (Jonathan
- Manage correctly an exec brute force attack avoiding the bypass (Valdis
- Other minor changes and cleanups.

Any constructive comments are welcome.
Thanks in advance.

John Wood (7):
   security: Add LSM hook at the point where a task gets a fatal signal
   security/brute: Define a LSM and add sysctl attributes
   security/brute: Detect a brute force attack
   security/brute: Mitigate a brute force attack
   selftests/brute: Add tests for the Brute LSM
   Documentation: Add documentation for the Brute LSM
   MAINTAINERS: Add a new entry for the Brute LSM

  Documentation/admin-guide/LSM/Brute.rst  | 334 +++++++++++
  Documentation/admin-guide/LSM/index.rst  |   1 +
  MAINTAINERS                              |   7 +
  include/linux/lsm_hook_defs.h            |   1 +
  include/linux/lsm_hooks.h                |   4 +
  include/linux/security.h                 |   4 +
  include/uapi/linux/xattr.h               |   3 +
  kernel/signal.c                          |   1 +
  security/Kconfig                         |  11 +-
  security/Makefile                        |   2 +
  security/brute/Kconfig                   |  15 +
  security/brute/Makefile                  |   2 +
  security/brute/brute.c                   | 716 +++++++++++++++++++++++
  security/security.c                      |   5 +
  tools/testing/selftests/Makefile         |   1 +
  tools/testing/selftests/brute/.gitignore |   2 +
  tools/testing/selftests/brute/Makefile   |   5 +
  tools/testing/selftests/brute/config     |   1 +
  tools/testing/selftests/brute/rmxattr.c  |  34 ++
  tools/testing/selftests/brute/test.c     | 507 ++++++++++++++++
  tools/testing/selftests/brute/    | 256 ++++++++
  21 files changed, 1907 insertions(+), 5 deletions(-)
  create mode 100644 Documentation/admin-guide/LSM/Brute.rst
  create mode 100644 security/brute/Kconfig
  create mode 100644 security/brute/Makefile
  create mode 100644 security/brute/brute.c
  create mode 100644 tools/testing/selftests/brute/.gitignore
  create mode 100644 tools/testing/selftests/brute/Makefile
  create mode 100644 tools/testing/selftests/brute/config
  create mode 100644 tools/testing/selftests/brute/rmxattr.c
  create mode 100644 tools/testing/selftests/brute/test.c
  create mode 100755 tools/testing/selftests/brute/


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