diff mbox series

[v2,7/8] Documentation: Add documentation for the Brute LSM

Message ID 20201025134540.3770-8-john.wood@gmx.com
State New
Headers show
Series Fork brute force attack mitigation | expand

Commit Message

John Wood Oct. 25, 2020, 1:45 p.m. UTC
Add some info detailing what is the Brute LSM, its motivation, weak
points of existing implementations, proposed solutions, enabling,
disabling and fine tuning.

Signed-off-by: John Wood <john.wood@gmx.com>
---
 Documentation/admin-guide/LSM/Brute.rst | 118 ++++++++++++++++++++++++
 Documentation/admin-guide/LSM/index.rst |   1 +
 security/brute/Kconfig                  |   3 +-
 3 files changed, 121 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/admin-guide/LSM/Brute.rst

--
2.25.1

Comments

Randy Dunlap Nov. 9, 2020, 4:31 a.m. UTC | #1
On 10/25/20 6:45 AM, John Wood wrote:
> Add some info detailing what is the Brute LSM, its motivation, weak
> points of existing implementations, proposed solutions, enabling,
> disabling and fine tuning.
> 
> Signed-off-by: John Wood <john.wood@gmx.com>
> ---
>  Documentation/admin-guide/LSM/Brute.rst | 118 ++++++++++++++++++++++++
>  Documentation/admin-guide/LSM/index.rst |   1 +
>  security/brute/Kconfig                  |   3 +-
>  3 files changed, 121 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..20c6ccbd625d
> --- /dev/null
> +++ b/Documentation/admin-guide/LSM/Brute.rst
> @@ -0,0 +1,118 @@
> +.. 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 traditionaly use some level of brute force with the help of

                      traditionally

> +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

                                                                           delaying

> +the fork system call if a child died due to a fatal error. This has some issues:
> +
> +Bad practices
> +-------------
> +
> +Add delays to the kernel is, in general, a bad idea.

   Adding

> +
> +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.
> +
> +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.

                          repeating

> +
> +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.

                   involved                         using

> +
> +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

                                                           consecutive

> +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

        way                                                               tune

> +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.
> +
> +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
> +
> +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)  -> To enable the feature
> +prctl(PR_SECURITY_FORK_BRUTE_DISABLE, 0, 0, 0, 0) -> To disable the feature
> +
> +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 an app could read crash_period_threshold and just do a new fork every
threshold + 1 time units, right? and not be caught?

thanks for the documentation.
John Wood Nov. 9, 2020, 6:23 p.m. UTC | #2
Hi,
Thanks for the typos corrections. Will be corrected in the next patch
version.

On Sun, Nov 08, 2020 at 08:31:13PM -0800, Randy Dunlap wrote:
>
> So an app could read crash_period_threshold and just do a new fork every
> threshold + 1 time units, right? and not be caught?

Yes, you are right. But we must set a crash_period_threshold that does not
make an attack feasible. For example, with the default value of 30000 ms,
an attacker can break the app only once every 30 seconds. So, to guess
canaries or break ASLR, the attack needs a big amount of time. But it is
possible.

So, I think that to avoid this scenario we can add a maximum number of
faults per fork hierarchy. Then, the mitigation will be triggered if the
application crash period falls under the period threshold or if the number
of faults exceed the maximum commented.

This way, if an attack is of long duration, it will also be detected and
mitigated.

What do you think?

>
> thanks for the documentation.
> --
> ~Randy
>

Thanks,
John Wood
Randy Dunlap Nov. 10, 2020, 12:09 a.m. UTC | #3
On 11/9/20 10:23 AM, John Wood wrote:
> Hi,
> Thanks for the typos corrections. Will be corrected in the next patch
> version.
> 
> On Sun, Nov 08, 2020 at 08:31:13PM -0800, Randy Dunlap wrote:
>>
>> So an app could read crash_period_threshold and just do a new fork every
>> threshold + 1 time units, right? and not be caught?
> 
> Yes, you are right. But we must set a crash_period_threshold that does not
> make an attack feasible. For example, with the default value of 30000 ms,
> an attacker can break the app only once every 30 seconds. So, to guess
> canaries or break ASLR, the attack needs a big amount of time. But it is
> possible.
> 
> So, I think that to avoid this scenario we can add a maximum number of
> faults per fork hierarchy. Then, the mitigation will be triggered if the
> application crash period falls under the period threshold or if the number
> of faults exceed the maximum commented.
> 
> This way, if an attack is of long duration, it will also be detected and
> mitigated.
> 
> What do you think?

Hi,
That sounds reasonable to me.

thanks.
diff mbox series

Patch

diff --git a/Documentation/admin-guide/LSM/Brute.rst b/Documentation/admin-guide/LSM/Brute.rst
new file mode 100644
index 000000000000..20c6ccbd625d
--- /dev/null
+++ b/Documentation/admin-guide/LSM/Brute.rst
@@ -0,0 +1,118 @@ 
+.. 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 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:
+
+Bad practices
+-------------
+
+Add delays to the kernel is, in general, a bad idea.
+
+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.
+
+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.
+
+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.
+
+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
+
+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)  -> To enable the feature
+prctl(PR_SECURITY_FORK_BRUTE_DISABLE, 0, 0, 0, 0) -> To disable the feature
+
+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.
+
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 in
+	  Documentation/admin-guide/LSM/Brute.rst.

 	  If you are unsure how to answer this question, answer N.