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Subject[RESEND][RFC PATCH 0/6] Fork brute force attack mitigation (fbfam)
[kees: re-sending this series on behalf of John Wood <>
also visible at fbfam]

From: John Wood <>

The goal of this patch serie is to detect and mitigate a fork brute force

Attacks 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 implementation.

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

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. So, 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 processes that fork the task 0, and all the processes that fork
after an execve system call.

These statistics hold the timestamp for the first fork (case of a fork of
task zero) or the timestamp for the execve system call (the other case).
Also, hold the number of faults of all the tasks that share the same
statistical data since the commented timestamp.

With this information it is possible to detect a brute force attack when a
task die in a fatal way computing the crashing rate. This rate shows the
milliseconds per fault and when it goes 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.

The fbfam feature can be enabled, disabled and tuned as follows:

1.- Per system enabling: This feature can be enabled in build time using
the config application under:

Security options ---> Fork brute force attack 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_FBFAM_ENABLE, 0, 0, 0, 0) -> To enable the feature
prctl(PR_FBFAM_DISABLE, 0, 0, 0, 0) -> To disable the feature

Both functions return zero on success and -EFAULT if the current task
doesn't have statistical data.

3.- Fine tuning: To customize the detection's sensibility there is a new
sysctl that allows to set the crashing rate threshold. It is accessible
through the file:


The units are in milliseconds per fault and the attack's mitigation is
triggered if the crashing rate of an application goes under this
threshold. So, the higher this value, the faster an attack will be

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

The 1/9 patch adds a new config for the fbfam feature.

The 2/9 and 3/9 patches add and use the api to manage the statistical data
necessary to compute the crashing rate of an application.

The 4/9 patch adds a new sysctl to fine tuning the detection's sensibility.

The 5/9 patch detects a fork brute force attack calculating the crashing

The 6/9 patch mitigates the attack killing all the offending tasks.

The 7/9 patch adds two new prctls to allow per task enabling/disabling.

The 8/9 patch adds general documentation.

The 9/9 patch adds an entry to the maintainers list.

This patch series is a task of the KSPP [1] and it is worth to mention
that there is a previous attempt without any continuation [2].


Any constructive comments are welcome.

Note: During the compilation these warnings were shown:

kernel/exit.o: warning: objtool: __x64_sys_exit_group()+0x18: unreachable instruction
arch/x86/kernel/cpu/mce/core.o: warning: objtool: mce_panic()+0x123: unreachable instruction
arch/x86/kernel/smpboot.o: warning: objtool: native_play_dead()+0x122: unreachable instruction
net/core/skbuff.o: warning: objtool: skb_push.cold()+0x14: unreachable instruction

John Wood (6):
security/fbfam: Add a Kconfig to enable the fbfam feature
security/fbfam: Add the api to manage statistics
security/fbfam: Use the api to manage statistics
security/fbfam: Add a new sysctl to control the crashing rate
security/fbfam: Detect a fork brute force attack
security/fbfam: Mitigate a fork brute force attack

fs/coredump.c | 2 +
fs/exec.c | 2 +
include/fbfam/fbfam.h | 24 ++++
include/linux/sched.h | 4 +
kernel/exit.c | 2 +
kernel/fork.c | 4 +
kernel/sysctl.c | 9 ++
security/Kconfig | 1 +
security/Makefile | 4 +
security/fbfam/Kconfig | 10 ++
security/fbfam/Makefile | 3 +
security/fbfam/fbfam.c | 279 ++++++++++++++++++++++++++++++++++++++++
security/fbfam/sysctl.c | 20 +++
13 files changed, 364 insertions(+)
create mode 100644 include/fbfam/fbfam.h
create mode 100644 security/fbfam/Kconfig
create mode 100644 security/fbfam/Makefile
create mode 100644 security/fbfam/fbfam.c
create mode 100644 security/fbfam/sysctl.c


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