[PATCH v2 0/8] Fork brute force attack mitigation

[John Wood]

Attacks against vulnerable userspace applications with the purpose to break
ASLR or bypass canaries traditionaly 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.

Other implementations
---------------------

The public version of grsecurity, as a summary, is based on the idea of
delay the fork system call if a child died due to a fatal error. This has
some issues:

1.- Bad practices: Add delays to the kernel is, in general, a bad idea.

2.- Weak points: This protection can be bypassed using two different
    methods since it acts only when the fork is called after a child has
    crashed.

    2.1.- Bypass 1: So, it would still be possible for an attacker to fork
          a big amount of children (in the order of thousands), then probe
          all of them, and finally wait the protection time before repeat
          the steps.

    2.2.- Bypass 2: This method is based on the idea that the protection
          doesn't act if the parent crashes. So, it would still be possible
          for an attacker to fork a process and probe itself. Then, fork
          the child process and probe itself again. This way, these steps
          can be repeated infinite times without any mitigation.

This implementation
-------------------

The main idea behind this implementation is to improve the existing ones
focusing on the weak points annotated before. The solution for the first
bypass method is to detect a fast crash rate instead of only one simple
crash. For the second bypass method the solution is to detect both the
crash of parent and child processes. Moreover, as a mitigation method it is
better to kill all the offending tasks involve in the attack instead of use
delays.

So, the solution to the two bypass methods previously commented is to use
some statistical data shared across all the processes that can have the
same memory contents. Or in other words, a statistical data shared between
all the fork hierarchy processes after an execve system call.

The purpose of these statistics is to compute the application crash period
in order to detect an attack. This crash period is the time between the
execve system call and the first fault or the time between two consecutives
faults, but this has a drawback. If an application crashes once quickly
from the execve system call or crashes twice in a short period of time for
some reason, a false positive attack will be triggered. To avoid this
scenario the shared statistical data holds a list of the i last crashes
timestamps and the application crash period is computed as follows:

crash_period = (n_last_timestamp - n_minus_i_timestamp) / i;

This ways, the size of the last crashes timestamps list allows to fine
tuning the detection sensibility.

When this crash period falls under a certain threshold there is a clear
signal that something malicious is happening. Once detected, the mitigation
only kills the processes that share the same statistical data and so, all
the tasks that can have the same memory contents. This way, an attack is
rejected.

1.- Per system enabling: This feature can be enabled at build time using
    the CONFIG_SECURITY_FORK_BRUTE option or using the visual config
    application under the following menu:

    Security options ---> Fork brute force attack detection and mitigation

2.- Per process enabling/disabling: To allow that specific applications can
    turn off or turn on the detection and mitigation of a fork brute force
    attack when required, there are two new prctls.

    prctl(PR_SECURITY_FORK_BRUTE_ENABLE, 0, 0, 0, 0)
    prctl(PR_SECURITY_FORK_BRUTE_DISABLE, 0, 0, 0, 0)

3.- Fine tuning: To customize the detection's sensibility there are two new
    sysctl attributes that allow to set the last crashes timestamps list
    size and the application crash period threshold (in milliseconds). Both
    are accessible through the following files respectively.

    /proc/sys/kernel/brute/timestamps_list_size
    /proc/sys/kernel/brute/crash_period_threshold

    The list size allows to avoid false positives due to crashes unrelated
    with a real attack. The period threshold sets the time limit to detect
    an attack. And, since a fork brute force attack will be detected if the
    application crash period falls under this threshold, the higher this
    value, the more sensitive the detection will be.

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 adds the sysctl attributes to fine tuning the detection.

Patchs 4/8 and 5/8 detect and mitigate a fork brute force attack.

Patch 6/8 adds the prctls to allow per process enabling/disabling.

Patch 7/8 adds the documentation to explain this implementation.

Patch 8/8 updates the maintainers file.

This patch series is a task of the KSPP [1] and can also be accessed from
my github tree [2] in the "brute_v2" branch.

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

The first version can be found in:

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

Changelog RFC -> v2
-------------------
- Rename this feature with a more appropiate 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).

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
  security/brute: Add sysctl attributes to allow detection fine tuning
  security/brute: Detect a fork brute force attack
  security/brute: Mitigate a fork brute force attack
  security/brute: Add prctls to enable/disable the fork attack detection
  Documentation: Add documentation for the Brute LSM
  MAINTAINERS: Add a new entry for the Brute LSM

 Documentation/admin-guide/LSM/Brute.rst | 118 ++++
 Documentation/admin-guide/LSM/index.rst |   1 +
 MAINTAINERS                             |   7 +
 include/brute/brute.h                   |  16 +
 include/linux/lsm_hook_defs.h           |   1 +
 include/linux/lsm_hooks.h               |   4 +
 include/linux/security.h                |   4 +
 include/uapi/linux/prctl.h              |   4 +
 kernel/signal.c                         |   1 +
 kernel/sys.c                            |   8 +
 security/Kconfig                        |  11 +-
 security/Makefile                       |   4 +
 security/brute/Kconfig                  |  13 +
 security/brute/Makefile                 |   2 +
 security/brute/brute.c                  | 749 ++++++++++++++++++++++++
 security/security.c                     |   5 +
 16 files changed, 943 insertions(+), 5 deletions(-)
 create mode 100644 Documentation/admin-guide/LSM/Brute.rst
 create mode 100644 include/brute/brute.h
 create mode 100644 security/brute/Kconfig
 create mode 100644 security/brute/Makefile
 create mode 100644 security/brute/brute.c

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2.25.1