On Sun, Mar 07, 2021 at 12:30:30PM +0100, John Wood wrote:
Add some info detailing what is the Brute LSM, its motivation, weak points of existing implementations, proposed solutions, enabling, disabling and self-tests.
Signed-off-by: John Wood john.wood@gmx.com
Documentation/admin-guide/LSM/Brute.rst | 278 ++++++++++++++++++++++++ Documentation/admin-guide/LSM/index.rst | 1 + security/brute/Kconfig | 3 +- 3 files changed, 281 insertions(+), 1 deletion(-) create mode 100644 Documentation/admin-guide/LSM/Brute.rst
diff --git a/Documentation/admin-guide/LSM/Brute.rst b/Documentation/admin-guide/LSM/Brute.rst new file mode 100644 index 000000000000..ca80aef9aa67 --- /dev/null +++ b/Documentation/admin-guide/LSM/Brute.rst @@ -0,0 +1,278 @@ +.. SPDX-License-Identifier: GPL-2.0 +=========================================================== +Brute: Fork brute force attack detection and mitigation LSM +===========================================================
+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 implementation. 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 really needs to be detected are fork/exec brute force attacks that +cross any of the commented bounds.
+Other implementations +=====================
+The public version of grsecurity, as a summary, is based on the idea of delaying +the fork system call if a child died due to some fatal signal (SIGSEGV, SIGBUS, +SIGKILL or SIGILL). This has some issues:
+Bad practices +-------------
+Adding delays to the kernel is, in general, a bad idea.
+Scenarios not detected (false negatives) +----------------------------------------
+This protection acts only when the fork system call is called after a child has +crashed. 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 repeating the steps.
+Moreover, 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.
+Scenarios detected (false positives) +------------------------------------
+Scenarios where an application rarely fails for reasons unrelated to a real +attack.
+This implementation +===================
+The main idea behind this implementation is to improve the existing ones +focusing on the weak points annotated before. Basically, the adopted solution is +to detect a fast crash rate instead of only one simple crash and to detect both +the crash of parent and child processes. Also, fine tune the detection focusing +on privilege boundary crossing. And finally, as a mitigation method, kill all +the offending tasks involved in the attack instead of using delays.
+To achieve this goal, and going into more details, this implementation is based +on the use of 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, basically, collect all the necessary info +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 consecutive faults, but this has a drawback. If an application +crashes twice in a short period of time for some reason unrelated to a real +attack, a false positive will be triggered. To avoid this scenario the +exponential moving average (EMA) is used. This way, the application crash period +will be a value that is not prone to change due to spurious data and follows the +real crash period.
+To detect a brute force attack it is necessary that the statistics shared by all +the fork hierarchy processes be updated in every fatal crash and the most +important data to update is the application crash period.
+These statistics are hold by the brute_stats struct.
+struct brute_cred {
- kuid_t uid;
- kgid_t gid;
- kuid_t suid;
- kgid_t sgid;
- kuid_t euid;
- kgid_t egid;
- kuid_t fsuid;
- kgid_t fsgid;
+};
+struct brute_stats {
- spinlock_t lock;
- refcount_t refc;
- unsigned char faults;
- u64 jiffies;
- u64 period;
- struct brute_cred saved_cred;
- unsigned char network : 1;
- unsigned char bounds_crossed : 1;
+};
Instead of open-coding this, just use the kerndoc references you've already built in the .c files:
.. kernel-doc:: security/brute/brute.c
+This is a fixed sized struct, so the memory usage will be based on the current +number of processes exec()ing. The previous sentence is true since in every fork +system call the parent's statistics are shared with the child process and in +every execve system call a new brute_stats struct is allocated. So, only one +brute_stats struct is used for every fork hierarchy (hierarchy of processes from +the execve system call).
+There are two types of brute force attacks that need to be detected. The first +one is an attack that happens through the fork system call and the second one is +an attack that happens through the execve system call. The first type uses the +statistics shared by all the fork hierarchy processes, but the second type +cannot use this statistical data due to these statistics dissapear when the +involved tasks finished. In this last scenario the attack info should be tracked +by the statistics of a higher fork hierarchy (the hierarchy that contains the +process that forks before the execve system call).
+Moreover, these two attack types have two variants. A slow brute force attack +that is detected if a maximum number of faults per fork hierarchy is reached and +a fast brute force attack that is detected if the application crash period falls +below a certain threshold.
+Once an attack has been detected, this is mitigated killing all the offending +tasks involved. Or in other words, once an attack has been detected, this is +mitigated killing all the processes that share the same statistics (the stats +that show an slow or fast brute force attack).
+Fine tuning the attack detection +--------------------------------
+To avoid false positives during the attack detection it is necessary to narrow +the possible cases. To do so, and based on the threat scenarios that we want to +detect, this implementation also focuses on the crossing of privilege bounds.
+To be precise, only the following privilege bounds are taken into account:
+1.- setuid/setgid process +2.- network to local +3.- privilege changes
+Moreover, only the fatal signals delivered by the kernel are taken into account +avoiding the fatal signals sent by userspace applications (with the exception of +the SIGABRT user signal since this is used by glibc for stack canary, malloc, +etc. failures, which may indicate that a mitigation has been triggered).
+Exponential moving average (EMA) +--------------------------------
+This kind of average defines a weight (between 0 and 1) for the new value to add +and applies the remainder of the weight to the current average value. This way, +some spurious data will not excessively modify the average and only if the new +values are persistent, the moving average will tend towards them.
+Mathematically the application crash period's EMA can be expressed as follows:
+period_ema = period * weight + period_ema * (1 - weight)
+Related to the attack detection, the EMA must guarantee that not many crashes +are needed. To demonstrate this, the scenario where an application has been +running without any crashes for a month will be used.
+The period's EMA can be written now as:
+period_ema[i] = period[i] * weight + period_ema[i - 1] * (1 - weight)
+If the new crash periods have insignificant values related to the first crash +period (a month in this case), the formula can be rewritten as:
+period_ema[i] = period_ema[i - 1] * (1 - weight)
+And by extension:
+period_ema[i - 1] = period_ema[i - 2] * (1 - weight) +period_ema[i - 2] = period_ema[i - 3] * (1 - weight) +period_ema[i - 3] = period_ema[i - 4] * (1 - weight)
+So, if the substitution is made:
+period_ema[i] = period_ema[i - 1] * (1 - weight) +period_ema[i] = period_ema[i - 2] * pow((1 - weight) , 2) +period_ema[i] = period_ema[i - 3] * pow((1 - weight) , 3) +period_ema[i] = period_ema[i - 4] * pow((1 - weight) , 4)
+And in a more generic form:
+period_ema[i] = period_ema[i - n] * pow((1 - weight) , n)
+Where n represents the number of iterations to obtain an EMA value. Or in other +words, the number of crashes to detect an attack.
+So, if we isolate the number of crashes:
+period_ema[i] / period_ema[i - n] = pow((1 - weight), n) +log(period_ema[i] / period_ema[i - n]) = log(pow((1 - weight), n)) +log(period_ema[i] / period_ema[i - n]) = n * log(1 - weight) +n = log(period_ema[i] / period_ema[i - n]) / log(1 - weight)
+Then, in the commented scenario (an application has been running without any +crashes for a month), the approximate number of crashes to detect an attack +(using the implementation values for the weight and the crash period threshold) +is:
+weight = 7 / 10 +crash_period_threshold = 30 seconds
+n = log(crash_period_threshold / seconds_per_month) / log(1 - weight) +n = log(30 / (30 * 24 * 3600)) / log(1 - 0.7) +n = 9.44
+So, with 10 crashes for this scenario an attack will be detected. If these steps +are repeated for different scenarios and the results are collected:
+1 month without any crashes ----> 9.44 crashes to detect an attack +1 year without any crashes -----> 11.50 crashes to detect an attack +10 years without any crashes ---> 13.42 crashes to detect an attack
+However, this computation has a drawback. The first data added to the EMA not +obtains a real average showing a trend. So the solution is simple, the EMA needs +a minimum number of data to be able to be interpreted. This way, the case where +a few first faults are fast enough followed by no crashes is avoided.
+Per system enabling/disabling +-----------------------------
+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
+Also, at boot time, this feature can be disable too, by changing the "lsm=" boot +parameter.
+Kernel selftests +----------------
+To validate all the expectations about this implementation, there is a set of +selftests. This tests cover fork/exec brute force attacks crossing the following +privilege boundaries:
+1.- setuid process +2.- privilege changes +3.- network to local
+Also, there are some tests to check that fork/exec brute force attacks without +crossing any privilege boundariy already commented doesn't trigger the detection +and mitigation stage.
+To build the tests: +make -C tools/testing/selftests/ TARGETS=brute
+To run the tests: +make -C tools/testing/selftests TARGETS=brute run_tests
+To package the tests: +make -C tools/testing/selftests TARGETS=brute gen_tar diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst index a6ba95fbaa9f..1f68982bb330 100644 --- a/Documentation/admin-guide/LSM/index.rst +++ b/Documentation/admin-guide/LSM/index.rst @@ -41,6 +41,7 @@ subdirectories. :maxdepth: 1
apparmor
- Brute LoadPin SELinux Smack
diff --git a/security/brute/Kconfig b/security/brute/Kconfig index 1bd2df1e2dec..334d7e88d27f 100644 --- a/security/brute/Kconfig +++ b/security/brute/Kconfig @@ -7,6 +7,7 @@ config SECURITY_FORK_BRUTE vulnerable userspace processes. The detection method is based on the application crash period and as a mitigation procedure all the offending tasks are killed. Like capabilities, this security module
stacks with other LSMs.
stacks with other LSMs. Further information can be found inDocumentation/admin-guide/LSM/Brute.rst.If you are unsure how to answer this question, answer N.
-- 2.25.1