| Message ID | 20210605150405.6936-1-john.wood@gmx.com |
|---|---|
| Headers | show |
| Series | Fork brute force attack mitigation | expand |
On Sat, Jun 05, 2021 at 05:03:57PM +0200, John Wood wrote: > [...] > the kselftest to avoid the detection ;) ). So, in this version, to track > all the statistical data (info related with application crashes), the > extended attributes feature for the executable files are used. The xattr is > also used to mark the executables as "not allowed" when an attack is > detected. Then, the execve system call rely on this flag to avoid following > executions of this file. I have some concerns about this being actually usable and not creating DoS situations. For example, let's say an attacker had found a hard-to-hit bug in "sudo", and starts brute forcing it. When the brute LSM notices, it'll make "sudo" unusable for the entire system, yes? And a reboot won't fix it, either, IIUC. It seems like there is a need to track "user" running "prog", and have that be timed out. Are there use-cases here where that wouldn't be sufficient? -Kees -- Kees Cook
On 6/8/2021 4:19 PM, Kees Cook wrote: > On Sat, Jun 05, 2021 at 05:03:57PM +0200, John Wood wrote: >> [...] >> the kselftest to avoid the detection ;) ). So, in this version, to track >> all the statistical data (info related with application crashes), the >> extended attributes feature for the executable files are used. The xattr is >> also used to mark the executables as "not allowed" when an attack is >> detected. Then, the execve system call rely on this flag to avoid following >> executions of this file. > I have some concerns about this being actually usable and not creating > DoS situations. For example, let's say an attacker had found a hard-to-hit > bug in "sudo", and starts brute forcing it. When the brute LSM notices, > it'll make "sudo" unusable for the entire system, yes? > > And a reboot won't fix it, either, IIUC. > The whole point of the mitigation is to trade potential attacks against DOS. If you're worried about DOS the whole thing is not for you. -Andi
On Tue, Jun 08, 2021 at 04:38:15PM -0700, Andi Kleen wrote: > > On 6/8/2021 4:19 PM, Kees Cook wrote: > > On Sat, Jun 05, 2021 at 05:03:57PM +0200, John Wood wrote: > > > [...] > > > the kselftest to avoid the detection ;) ). So, in this version, to track > > > all the statistical data (info related with application crashes), the > > > extended attributes feature for the executable files are used. The xattr is > > > also used to mark the executables as "not allowed" when an attack is > > > detected. Then, the execve system call rely on this flag to avoid following > > > executions of this file. > > I have some concerns about this being actually usable and not creating > > DoS situations. For example, let's say an attacker had found a hard-to-hit > > bug in "sudo", and starts brute forcing it. When the brute LSM notices, > > it'll make "sudo" unusable for the entire system, yes? > > > > And a reboot won't fix it, either, IIUC. > > > The whole point of the mitigation is to trade potential attacks against DOS. > > If you're worried about DOS the whole thing is not for you. Right, but there's no need to make a system unusable for everyone else. There's nothing here that relaxes the defense (i.e. stop spawning apache for 10 minutes). Writing it to disk with nothing that undoes it seems a bit too much. :) -- Kees Cook
On Wed, Jun 09, 2021 at 09:52:29AM -0700, Kees Cook wrote: > On Tue, Jun 08, 2021 at 04:38:15PM -0700, Andi Kleen wrote: > > > > On 6/8/2021 4:19 PM, Kees Cook wrote: > > > On Sat, Jun 05, 2021 at 05:03:57PM +0200, John Wood wrote: > > > > [...] > > > > the kselftest to avoid the detection ;) ). So, in this version, to track > > > > all the statistical data (info related with application crashes), the > > > > extended attributes feature for the executable files are used. The xattr is > > > > also used to mark the executables as "not allowed" when an attack is > > > > detected. Then, the execve system call rely on this flag to avoid following > > > > executions of this file. > > > > > > I have some concerns about this being actually usable and not creating > > > DoS situations. For example, let's say an attacker had found a hard-to-hit > > > bug in "sudo", and starts brute forcing it. When the brute LSM notices, > > > it'll make "sudo" unusable for the entire system, yes? > > > > > > And a reboot won't fix it, either, IIUC. > > > > > The whole point of the mitigation is to trade potential attacks against DOS. > > > > If you're worried about DOS the whole thing is not for you. > > Right, but there's no need to make a system unusable for everyone else. > There's nothing here that relaxes the defense (i.e. stop spawning apache > for 10 minutes). Writing it to disk with nothing that undoes it seems a > bit too much. :) Here I have merge the first reply. > It seems like there is a need to track "user" running "prog", and have > that be timed out. Are there use-cases here where that wouldn't be > sufficient? Ok, what do you think of the following proposal: Add an uid_t field to the structure saved in the xattr. So this struct contains now faults: Number of crashes. nsecs: Last crash timestamp as the number of nanoseconds in the International Atomic Time (TAI) reference. period: Crash period's moving average. flags: Statistics flags. uid: User id not allowed to run the executable. The logic would be the following: 1. faults, nsecs and period are updated in every crash and is a common info for all the users. 2. If the max number of faults is reached, it is "not allowed" to run the executable by any user. This condition blocks the file until root clear the xattr. No timeout. 3. When an attack is detected the uid of the user that is running the app is saved in the xattr and the executable is marked as "not allowed" to run by this user. The "not allowed" state has a timeout (more below). 4. When someone tries to run the executable, if his uid is different from the uid saved in the xattr, then the operation is "allowed". 5. When someone tries to run the executable, if his uid is equal to the uid saved in the xattr, then the operation is "not allowed". This user is banned for a timeout. 6. When someone tries to run the executable and the timeout has expired, the operation is "allowed" and the saved uid is removed. 7. If the executable crashes again when it is run by a user different from the one saved in the xattr (and the timeout has no expired), the file is marked as "not allowed" to run by any user. All users are banned for a timeout. The timeout: I think there are two options here. 1. A fixed timeout set by a sysctl attribute. 2. A dynamic timeout calculated from the info stored in the xattr. The timeout would be the needed period to guarantee that when the app is run again and it crashes, the attack detection will not be triggered. To be more clear I expose the formulas: Mathematically the application crash period's EMA can be expressed as follows: period_ema[i] = period[i] * weight + period_ema[i - 1] * (1 - weight) If we isolate period: period[i] = (period_ema[i] - period_ema[i - 1] * (1 - weight)) / weight Where period_ema[i] is the "crash_period_threshold", period_ema[i - 1] is the last period ema saved in the xattr and period[i] is the dynamic timeout. As a final point. Possibly there are more cases but the logic would be the one explained. I think that it is not necessary to save the uid for every user that crashes the app nor the crashes info for every user. If more than one user crashes the application, something "bad" is happening. So, all users are banned for a timeout. This way the info saved in the xattr has a fixed size and we prevent an attacker from abusing this size. I hope this proposal can be enough. What do you think? John Wood.
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 patch serie. 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 will really be detected are fork/exec brute force attacks that cross any of the commented bounds. The implementation details and comparison against other existing implementations can be found in the "Documentation" patch. It is important to mention that the v8 and v7 versions have changed the method used to track the information related to the application crashes. Prior this versions, a pointer per process (in the task_struct structure) held a reference to the shared statistical data. Or in other words, these stats were shared by all the fork hierarchy processes. But this has an important drawback: a brute force attack that happens through the execve system call losts the faults info since these statistics are freed when the fork hierarchy disappears. So, the solution adopted in the v6 version was to use an upper fork hierarchy to track the info for this attack type. But, as Valdis Kletnieks pointed out during this discussion [1], this method can be easily bypassed using a double exec (well, this was the method used in the kselftest to avoid the detection ;) ). So, in this version, to track all the statistical data (info related with application crashes), the extended attributes feature for the executable files are used. The xattr is also used to mark the executables as "not allowed" when an attack is detected. Then, the execve system call rely on this flag to avoid following executions of this file. [1] https://lore.kernel.org/kernelnewbies/20210330173459.GA3163@ubuntu/ Moreover, I think this solves another problem pointed out by Andi Kleen during the v5 review [2] related to the possibility that a supervisor respawns processes killed by the Brute LSM. He suggested adding some way so a supervisor can know that a process has been killed by Brute and then decide to respawn or not. So, now, the supervisor can read the brute xattr of one executable and know if it is blocked by Brute and why (using the statistical data). [2] https://lore.kernel.org/kernel-hardening/878s78dnrm.fsf@linux.intel.com/ Although the xattr of the executable is accessible from userspace, in complex daemons this file may not be visible directly by the supervisor as it may be run through some wrapper. So, an extension to the waitid() system call has been added in this version. This was suggested by Andi Kleen [3] during the v7 review. (The case with supervisors using cgroups is not yet tested). [3] https://lore.kernel.org/kernel-hardening/19903478-52e0-3829-0515-3e17669108f7@linux.intel.com/ 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 the necessary sysctl attributes to fine tuning the attack detection. The 3/8 patch detects a fork/exec brute force attack and narrows the possible cases taken into account the privilege boundary crossing. The 4/8 patch mitigates a brute force attack. The 5/8 patch adds the extension to the waitid system call to notify to userspace that a task has been killed by Brute LSM when an attack is mitigated. The 6/8 patch adds self-tests to validate the Brute LSM expectations. The 7/8 patch adds the documentation to explain this implementation. The 8/8 patch updates the maintainers file. This patch serie is a task of the KSPP [4] and can also be accessed from my github tree [5] in the "brute_v8" branch. [4] https://github.com/KSPP/linux/issues/39 [5] https://github.com/johwood/linux/ When I ran the "checkpatch" script I got the following errors, but I think they are false positives as I follow the same coding style for the others extended attributes suffixes. ---------------------------------------------------------------------------- ../patches/brute_v8/v8-0003-security-brute-Detect-a-brute-force-attack.patch ---------------------------------------------------------------------------- ERROR: Macros with complex values should be enclosed in parentheses 89: FILE: include/uapi/linux/xattr.h:80: +#define XATTR_NAME_BRUTE XATTR_SECURITY_PREFIX XATTR_BRUTE_SUFFIX ----------------------------------------------------------------------------- ../patches/brute_v8/v8-0006-selftests-brute-Add-tests-for-the-Brute-LSM.patch ----------------------------------------------------------------------------- ERROR: Macros with complex values should be enclosed in parentheses 159: FILE: tools/testing/selftests/brute/rmxattr.c:18: +#define XATTR_NAME_BRUTE XATTR_SECURITY_PREFIX XATTR_BRUTE_SUFFIX When I ran the "kernel-doc" script with the following parameters: ./scripts/kernel-doc --none -v security/brute/brute.c I got the following warning: security/brute/brute.c:118: warning: contents before sections But I don't understand why it is complaining. Could it be a false positive? The previous versions can be found in: RFC https://lore.kernel.org/kernel-hardening/20200910202107.3799376-1-keescook@chromium.org/ Version 2 https://lore.kernel.org/kernel-hardening/20201025134540.3770-1-john.wood@gmx.com/ Version 3 https://lore.kernel.org/lkml/20210221154919.68050-1-john.wood@gmx.com/ Version 4 https://lore.kernel.org/lkml/20210227150956.6022-1-john.wood@gmx.com/ Version 5 https://lore.kernel.org/kernel-hardening/20210227153013.6747-1-john.wood@gmx.com/ Version 6 https://lore.kernel.org/kernel-hardening/20210307113031.11671-1-john.wood@gmx.com/ Version 7 https://lore.kernel.org/kernel-hardening/20210521172414.69456-1-john.wood@gmx.com/ Changelog RFC -> v2 ------------------- - Rename this feature with a more suitable 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). Changelog v2 -> v3 ------------------ - Compute the application crash period on an on-going basis (Kees Cook). - Detect a brute force attack through the execve system call (Kees Cook). - Detect an slow brute force attack (Randy Dunlap). - Fine tuning the detection taken into account privilege boundary crossing (Kees Cook). - Taken into account only fatal signals delivered by the kernel (Kees Cook). - Remove the sysctl attributes to fine tuning the detection (Kees Cook). - Remove the prctls to allow per process enabling/disabling (Kees Cook). - Improve the documentation (Kees Cook). - Fix some typos in the documentation (Randy Dunlap). - Add self-test to validate the expectations (Kees Cook). Changelog v3 -> v4 ------------------ - Fix all the warnings shown by the tool "scripts/kernel-doc" (Randy Dunlap). Changelog v4 -> v5 ------------------ - Fix some typos (Randy Dunlap). Changelog v5 -> v6 ------------------ - Fix a reported deadlock (kernel test robot). - Add high level details to the documentation (Andi Kleen). Changelog v6 -> v7 ------------------ - Add the "Reviewed-by:" tag to the first patch. - Rearrange the brute LSM between lockdown and yama (Kees Cook). - Split subdir and obj in security/Makefile (Kees Cook). - Reduce the number of header files included (Kees Cook). - Print the pid when an attack is detected (Kees Cook). - Use the socket_accept LSM hook instead of socket_sock_rcv_skb hook to avoid running a hook on every incoming network packet (Kees Cook). - Update the documentation and fix it to render it properly (Jonathan Corbet). - Manage correctly an exec brute force attack avoiding the bypass (Valdis Kletnieks). - Other minor changes and cleanups. Changelog v7 -> v8 ------------------ - Rebase against v5.13-rc4. - Fix a build error if CONFIG_IPV6 and/or CONFIG_SECURITY_NETWORK is not set (kernel test robot). - Notify to userspace that a task has been killed by Brute LSM (Andi Kleen). - Add a new test to verify that the userspace notification is working. - Update the documentation accordingly with this new feature. - Other minor changes and cleanups. Any constructive comments are welcome. Thanks in advance. John Wood (8): security: Add LSM hook at the point where a task gets a fatal signal security/brute: Define a LSM and add sysctl attributes security/brute: Detect a brute force attack security/brute: Mitigate a brute force attack security/brute: Notify to userspace "task killed" selftests/brute: Add tests for the Brute LSM Documentation: Add documentation for the Brute LSM MAINTAINERS: Add a new entry for the Brute LSM Documentation/admin-guide/LSM/Brute.rst | 359 ++++++++++ Documentation/admin-guide/LSM/index.rst | 1 + MAINTAINERS | 8 + arch/x86/kernel/signal_compat.c | 2 +- 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/asm-generic/siginfo.h | 3 +- include/uapi/linux/xattr.h | 3 + kernel/exit.c | 6 +- kernel/signal.c | 5 +- security/Kconfig | 11 +- security/Makefile | 2 + security/brute/Kconfig | 15 + security/brute/Makefile | 2 + security/brute/brute.c | 795 +++++++++++++++++++++++ security/security.c | 5 + tools/testing/selftests/Makefile | 1 + tools/testing/selftests/brute/.gitignore | 3 + tools/testing/selftests/brute/Makefile | 5 + tools/testing/selftests/brute/config | 1 + tools/testing/selftests/brute/exec.c | 46 ++ tools/testing/selftests/brute/rmxattr.c | 34 + tools/testing/selftests/brute/test.c | 507 +++++++++++++++ tools/testing/selftests/brute/test.sh | 269 ++++++++ 26 files changed, 2099 insertions(+), 9 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 create mode 100644 tools/testing/selftests/brute/.gitignore create mode 100644 tools/testing/selftests/brute/Makefile create mode 100644 tools/testing/selftests/brute/config create mode 100644 tools/testing/selftests/brute/exec.c create mode 100644 tools/testing/selftests/brute/rmxattr.c create mode 100644 tools/testing/selftests/brute/test.c create mode 100755 tools/testing/selftests/brute/test.sh -- 2.25.1