Attacks against vulnerable userspace applications with the purpose to break ASLR or bypass canaries traditionally use some level of brute force with the help of the fork system call. This is possible since when creating a new process using fork its memory contents are the same as those of the parent process (the process that called the fork system call). So, the attacker can test the memory infinite times to find the correct memory values or the correct memory addresses without worrying about crashing the application.

Based on the above scenario it would be nice to have this detected and mitigated, and this is the goal of this patch serie. Specifically the following attacks are expected to be detected:

1.- Launching (fork()/exec()) a setuid/setgid process repeatedly until a desirable memory layout is got (e.g. Stack Clash). 2.- Connecting to an exec()ing network daemon (e.g. xinetd) repeatedly until a desirable memory layout is got (e.g. what CTFs do for simple network service). 3.- Launching processes without exec() (e.g. Android Zygote) and exposing state to attack a sibling. 4.- Connecting to a fork()ing network daemon (e.g. apache) repeatedly until the previously shared memory layout of all the other children is exposed (e.g. kind of related to HeartBleed).

In each case, a privilege boundary has been crossed:

Case 1: setuid/setgid process Case 2: network to local Case 3: privilege changes Case 4: network to local

So, what will really be detected are fork/exec brute force attacks that cross any of the commented bounds.

The implementation details and comparison against other existing implementations can be found in the "Documentation" patch.

This v4 version has changed a lot from the v2. Basically the application crash period is now compute on an on-going basis using an exponential moving average (EMA), a detection of a brute force attack through the "execve" system call has been added and the crossing of the commented privilege bounds are taken into account. Also, the fine tune has also been removed and now, all this kind of attacks are detected without administrator intervention.

In the v2 version Kees Cook suggested to study if the statistical data shared by all the fork hierarchy processes can be tracked in some other way. Specifically the question was if this info can be hold by the family hierarchy of the mm struct. After studying this hierarchy I think it is not suitable for the Brute LSM since they are totally copied on fork() and in this case we want that they are shared. So I leave this road.

So, knowing all this information I will explain now the different patches:

The 1/8 patch defines a new LSM hook to get the fatal signal of a task. This will be useful during the attack detection phase.

The 2/8 patch defines a new LSM and manages the statistical data shared by all the fork hierarchy processes.

The 3/8 patch detects a fork/exec brute force attack.

The 4/8 patch narrows the detection taken into account the privilege boundary crossing.

The 5/8 patch mitigates a brute force attack.

The 6/8 patch adds self-tests to validate the Brute LSM expectations.

The 7/8 patch adds the documentation to explain this implementation.

The 8/8 patch updates the maintainers file.

This patch serie is a task of the KSPP [1] and can also be accessed from my github tree [2] in the "brute_v4" branch.

[1] https://github.com/KSPP/linux/issues/39 [2] https://github.com/johwood/linux/

The previous versions can be found in:

RFC https://lore.kernel.org/kernel-hardening/20200910202107.3799376-1-keescook@c...

Version 2 https://lore.kernel.org/kernel-hardening/20201025134540.3770-1-john.wood@gmx...

Version 3 https://lore.kernel.org/lkml/20210221154919.68050-1-john.wood@gmx.com/

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 Cook). - 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 Cook). - 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 Dunlap).

Any constructive comments are welcome. Thanks.

John Wood (8): security: Add LSM hook at the point where a task gets a fatal signal security/brute: Define a LSM and manage statistical data securtiy/brute: Detect a brute force attack security/brute: Fine tuning the attack detection 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 | 224 +++++ 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 + kernel/signal.c | 1 + security/Kconfig | 11 +- security/Makefile | 4 + security/brute/Kconfig | 13 + security/brute/Makefile | 2 + security/brute/brute.c | 1102 ++++++++++++++++++++++ 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/exec.c | 44 + tools/testing/selftests/brute/test.c | 507 ++++++++++ tools/testing/selftests/brute/test.sh | 226 +++++ 20 files changed, 2160 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/exec.c create mode 100644 tools/testing/selftests/brute/test.c create mode 100755 tools/testing/selftests/brute/test.sh

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