Date: Sun, 25 Oct 2020 14:45:32 +0100 From: John Wood <john.wood@....com> To: Kees Cook <keescook@...omium.org>, Jann Horn <jannh@...gle.com> Cc: John Wood <john.wood@....com>, Jonathan Corbet <corbet@....net>, James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org, kernel-hardening@...ts.openwall.com Subject: [PATCH v2 0/8] Fork brute force attack mitigation 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  and can also be accessed from my github tree  in the "brute_v2" branch.  https://github.com/KSPP/linux/issues/39  https://github.com/johwood/linux/ The first version can be found in: https://firstname.lastname@example.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 -- 2.25.1
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