Hi List, This is v5 of automatic module load restriction series. I have renamed it to 'Improve Module autoloading infrastructure' for abvious reasons:
* It is more of an infrastructure change now. * Reduce the use of word 'restriction' and use 'improving' maybe easy to sell ? These patches are based on next-20171127 Credits: ======== The idea was inspired from grsecurity 'GRKERNSEC_MODHARDEN' config option. However, upstream Linux implementation is more focused on the run-time behavior with a three mode switch, plus upstream version solves Linux usecases with a per process tree flag that can be used in containers, sandboxes, etc to block direct implicit auto-load operations. This implementation does not share anything with grsecurity. The first RFC was as LSM, Kees Cook and others suggested that it can be turned as a core kernel feature and after some months, they were right. Previous versions: ================== v1 RFC as an LSM: http://www.openwall.com/lists/kernel-hardening/2017/02/02/21 v2 RFC as an LSM: http://www.openwall.com/lists/kernel-hardening/2017/04/09/1 v3 core feature as requested by kernel maintainers: https://lkml.org/lkml/2017/4/19/1086 v4: https://lkml.org/lkml/2017/5/22/312 All previous feedback from: Andy Lutomirski, Solar Designer, Kees Cook, Rusty Russell, Ben Hutchings and Serge Hallyn is fixed in this series. Please check Changelog for details. Thank you for the feedback. ============== Currently, an explicit call to load or unload kernel modules require CAP_SYS_MODULE capability. However unprivileged users have always been able to load some modules using the implicit auto-load operation. An automatic module loading happens when programs request a kernel feature from a module that is not loaded. In order to satisfy userspace, the kernel then automatically load all these required modules. Historically, the kernel was always able to automatically load modules if they are not blacklisted. This is one of the most important and transparent operations of Linux, it allows to provide numerous other features as they are needed which is crucial for a better user experience. However, as Linux is popular now and used for different appliances some of these may need to control such operations. For such systems, recent needs showed that in some cases allowing to control automatic module loading is as important as the operation itself. Restricting unprivileged programs or attackers that abuse this feature to load unused modules or modules that contain bugs is a significant security measure. This allows administrators or some special programs to have the appropriate time to update and deny module autoloading in advance, then blacklist the corresponding ones. Not doing so may affect the global state of the machine, especially containers where some apps are moved from one context to another and not having such mechanisms may allow to expose and exploit the vulnerable parts to escape the container sandbox. Embedded or IoT devices also started to ship as containers using generic distros, some vendors do not have the appropriate time to make their own OS, hence, using base images is getting popular. These setups may include unnecessary modules that the final applications will not need. Untrusted access may abuse the module auto-load feature to expose vulnerabilities. As every code contains bugs or vulnerabilties, the following vulnerabilities that affected some features that are often compiled as modules could have been completely blocked, by a better mechanism that handles module autoloading, especially if the system does not need them. Past months: * DCCP use after free CVE-2017-6074 [1] [2] Unprivileged to local root. * XFRM framework CVE-2017-7184 [3] As advertised it seems it was used to break Ubuntu on a security contest. * n_hldc CVE-2017-2636 [4] [5] Local privilege escalation. * L2TPv3 CVE-2016-10200 The list is longer. To improve the current status, this series tries to re-work how module autoloading is performed by adding two new properties: "modules_autoload_mode" sysctl flag, and a per-task one. The sysctl controls modules auto-load feature and complements "modules_disabled" which apply to all modules operations. This new flag allows to control only automatic module loading and if it is allowed or not, aligning in the process the implicit operation with the explicit one where both now are covered by capabilities checks. The three modes that "modules_autoload_mode" support allow to provide restrictions on automatic module loading without breaking user experience. The sysctl flag is available at "/proc/sys/kernel/modules_autoload_mode" When modules_autoload_mode is set to (0), the default, there are no restrictions. When modules_autoload_mode is set to (1), processes must have CAP_SYS_MODULE to be able to trigger a module auto-load operation, or CAP_NET_ADMIN for modules with a 'netdev-%s' alias, or other capabilities for specific aliased modules. When modules_autoload_mode is set to (2), automatic module loading is disabled for all. Notes on relation between "modules_disabled=0" and "modules_autoload_mode=2": 1) Once "modules_disabled=1" set, it needs a reboot to undo the setting. 2) Restricting automatic module loading does not interfere with explicit module load or unload operations. 3) New features provided by modules can be made available without rebooting the system. 4) A bad version of a module can be unloaded and replaced with a better one without rebooting the system. ========================== The patches also support process trees, containers, and sandboxes by providing an inherited per-task "modules_autoload_mode" flag that cannot be re-enabled once disabled. This offers the following advantages: 1) Automatic module loading is still available to the rest of the system. 2) It is easy to use in containers and sandboxes. DCCP example could have been used to escape containers. The XFRM framework CVE-2017-7184 needs CAP_NET_ADMIN, but attackers may start to target CAP_NET_ADMIN, a per-task flag will make it harder. 3) Suitable for desktop and more interactive Linux systems. 4) Will allow in future to implement a per user policy. The user database format is old and not extensible, as discussed maybe with a modern format we may achieve the following: User=djalal NewKernelFeatures=yes Which means that that interactive user will be allowed to load extra Linux features. Others, volatile accounts or guests can be easily blocked from doing so. 5) CAP_NET_ADMIN is useful, it handles lot of operations, at same time it started to look more like CAP_SYS_ADMIN which is overloaded. We need CAP_NET_ADMIN, containers need it, but at same time maybe we do not want programs running with it to load 'netdev-%s' modules. Having an extra per-task flag allow to discharge a bit CAP_NET_ADMIN and clearly target automatic module loading operations. Usage: prctl(PR_SET_MODULES_AUTOLOAD_mode, value, 0, 0, 0). The per-task "modules_autoload_mode" supports the following values: 0 There are no restrictions, usually the default unless set by parent. 1 The task must have CAP_SYS_MODULE to be able to trigger a module auto-load operation, or CAP_NET_ADMIN for modules with a 'netdev-%s' alias. 2 Automatic modules loading is disabled for the current task. The mode may only be increased, never decreased, thus ensuring that once applied, processes can never relax their setting. This make it easy for developers and users to handle. Note that even if the per-task "modules_autoload_mode" allows to auto-load the corresponding modules, automatic module loading may still fail due to the global sysctl "modules_autoload_mode". For more details please see Documentation/sysctl/kernel.txt, section "modules_autoload_mode". When a request to a kernel module is denied, the module name with the corresponding process name and its pid are logged. Administrators can use such information to explicitly load the appropriate modules. # Testing: ##) Global sysctl "modules_autoload_mode" Before patch: $ lsmod | grep ipip - $ sudo ip tunnel add mytun mode ipip remote 10.0.2.100 local 10.0.2.15 ttl 255 $ lsmod | grep ipip - ipip 16384 0 tunnel4 16384 1 ipip ip_tunnel 28672 1 ipip $ cat /proc/sys/kernel/modules_autoload_mode 0 After patch: $ lsmod | grep ipip - # echo 2 > /proc/sys/kernel/modules_autoload_mode $ sudo ip tunnel add mytun mode ipip remote 10.0.2.100 local 10.0.2.15 ttl 255 add tunnel "tunl0" failed: No such device $ dmesg ... [ 1876.378389] module: automatic module loading of netdev-tunl0 by "ip"[1453] was denied [ 1876.380994] module: automatic module loading of tunl0 by "ip"[1453] was denied ... $ lsmod | grep ipip - ##) Per-task "modules_autoload_mode" flag Here we use DCCP as an example since the public PoC was against it. The following tool can be used to test the feature: https://gist.githubusercontent.com/tixxdz/cf567e4275714199a32c4a80de4ea63a/raw/13e52ea0ee65772871bcf10fb6c94fedd349f5c1/pr_modules_autoload_mode_test.c DCCP use after free CVE-2017-6074 (unprivileged to local root): The code path can be triggered by unprivileged, using the trigger.c program for DCCP use after free [2] and that was fixed by commit 5edabca9d4cff7f "dccp: fix freeing skb too early for IPV6_RECVPKTINFO". Before patch: $ lsmod | grep dccp $ strace ./dccp_trigger ... socket(AF_INET6, SOCK_DCCP, IPPROTO_IP) = 3 ... $ lsmod | grep dccp dccp_ipv6 24576 5 dccp_ipv4 24576 5 dccp_ipv6 dccp 102400 2 dccp_ipv6,dccp_ipv4 $ grep Modules /proc/self/status ModulesAutoloadMode: 0 After: Set task "modules_autoload_mode" to 1, privileged mode. $ lsmod | grep dccp $ ./pr_set_no_new_privs $ grep NoNewPrivs /proc/self/status NoNewPrivs: 1 $ ./pr_modules_autoload_mode_test 1 $ grep Modules /proc/self/status ModulesAutoloadMode: 1 $ strace ./dccp_trigger ... socket(AF_INET6, SOCK_DCCP, IPPROTO_IP) = -1 ESOCKTNOSUPPORT (Socket type not supported) ... $ lsmod | grep dccp $ dmesg ... [ 4662.171994] module: automatic module loading of net-pf-10-proto-0-type-6 by "dccp_trigger"[1759] was denied [ 4662.177284] module: automatic module loading of net-pf-10-proto-0 by "dccp_trigger"[1759] was denied [ 4662.180181] module: automatic module loading of net-pf-10-proto-0-type-6 by "dccp_trigger"[1759] was denied [ 4662.181709] module: automatic module loading of net-pf-10-proto-0 by "dccp_trigger"[1759] was denied Now task "modules_autoload_mode" to 2, disabled mode. $ lsmod | grep dccp $ grep Modules /proc/self/status ModulesAutoloadMode: 0 $ su - root # ./pr_modules_autoload_mode_test 2 # grep Modules /proc/self/status ModulesAutoloadMode: 2 # strace ./dccp_trigger ... socket(AF_INET6, SOCK_DCCP, IPPROTO_IP) = -1 ESOCKTNOSUPPORT (Socket type not supported) ... ... [ 5154.218740] module: automatic module loading of net-pf-10-proto-0-type-6 by "dccp_trigger"[1873] was denied [ 5154.219828] module: automatic module loading of net-pf-10-proto-0 by "dccp_trigger"[1873] was denied [ 5154.221814] module: automatic module loading of net-pf-10-proto-0-type-6 by "dccp_trigger"[1873] was denied [ 5154.222731] module: automatic module loading of net-pf-10-proto-0 by "dccp_trigger"[1873] was denied As showed, this blocks automatic module loading per-task. This allows to provide a usable system, where only some sandboxed apps or containers will be restricted to trigger automatic module loading, other parts of the system can continue to use the feature as it is which is the case of the desktop and userfriendly machines. [1] http://www.openwall.com/lists/oss-security/2017/02/22/3 [2] https://github.com/xairy/kernel-exploits/tree/master/CVE-2017-6074 [3] http://www.openwall.com/lists/oss-security/2017/03/29/2 [4] http://www.openwall.com/lists/oss-security/2017/03/07/6 [5] https://a13xp0p0v.github.io/2017/03/24/CVE-2017-2636.html Finally we already have a use case for the prctl() interface to enforce some systemd services, in docker and other containers, also in some sandboxes, etc. # Changes since v4: *) Removed the property that when the "modules_autoload_mode" sysctl is set to "2" disabled mode, then that value is pinned and we can not revert it. Now you can undo the value if you have the appropriate privileges as it was suggested. Suggested-by: Solar Designer <so...@openwall.com> Suggested-by: Andy Lutomirski <l...@kernel.org> https://lkml.org/lkml/2017/5/22/330 *) Added request_module_cap() to take '@required_cap' and '@prefix' arguments that will be used to check if module autoloading is allowed or not. Suggested-by: Kees Cook <keesc...@chromium.org> *) More cleanups and documentation. # Changes since v3: *) Renamed the sysctl from "modules_autoload" to "modules_autoload_mode" and the prctl() operation flag to "PR_{SET|GET}_MODULES_AUTOLOAD_MODE" as it was requested. Suggested-by: Ben Hutchings <ben.hutchi...@codethink.co.uk> *) Updated __request_module() to take the capability that may allow to auto-load a module with the appropriate alias. This way we never parse aliases as it was requested by Rusty Russell. Security and SELinux hooks were updated too. Suggested-by: Rusty Russell <ru...@rustcorp.com.au> https://lkml.org/lkml/2017/4/24/7 *) Updated code to set prctl(PR_SET_MODULES_AUTOLOAD_MODE, 1, 0, 0, 0), the task must call prctl(PR_SET_NO_NEW_PRIVS, 1) before or run with CAP_SYS_ADMIN privileges in its namespace. If these are not true, -EACCES will be returned. Suggested-by: Andy Lutomirski <l...@amacapital.net> https://lkml.org/lkml/2017/4/22/22 *) Remove task initialization logic and other cleanups Suggested-by: Kees Cook <keesc...@chromium.org> *) Other code and documentation cleanups. # Changes since v2: *) Implemented as a core kernel feature inside capabilities subsystem *) Renamed sysctl to "modules_autoload" to align with "modules_disabled" Suggested-by: Kees Cook <keesc...@chromium.org> *) Improved documentation. *) Removed unused code. # Changes since v1: *) Renamed module to ModAutoRestrict *) Improved documentation to explicity refer to module autoloading. *) Switched to use the new task_security_alloc() hook. *) Switched from rhash tables to use task->security since it is in linux-security/next branch now. *) Check all parameters passed to prctl() syscall. *) Many other bug fixes and documentation improvements. Patches (5) Djalal Harouni: (1/5) modules:capabilities: add request_module_cap() (2/5) modules:capabilities: add cap_kernel_module_request() permission check (3/5) modules:capabilities: automatic module loading restriction (4/5) modules:capabilities: add a per-task modules auto-load mode (5/5) net: modules: use request_module_cap() to load 'netdev-%s' modules Documentation/filesystems/proc.txt | 3 + Documentation/sysctl/kernel.txt | 54 ++++++++ Documentation/userspace-api/index.rst | 1 + .../userspace-api/modules_autoload_mode.rst | 116 ++++++++++++++++ fs/proc/array.c | 6 + include/linux/init_task.h | 8 ++ include/linux/kmod.h | 65 ++++++++- include/linux/lsm_hooks.h | 6 +- include/linux/module.h | 41 +++++- include/linux/sched.h | 5 + include/linux/security.h | 11 +- include/uapi/linux/prctl.h | 8 ++ kernel/kmod.c | 29 +++- kernel/module.c | 153 +++++++++++++++++++++ kernel/sysctl.c | 28 ++++ net/core/dev_ioctl.c | 4 +- security/commoncap.c | 62 +++++++++ security/security.c | 6 +- security/selinux/hooks.c | 3 +- 19 files changed, 587 insertions(+), 22 deletions(-)
