diff options
Diffstat (limited to 'Documentation/admin-guide')
| -rw-r--r-- | Documentation/admin-guide/LSM/Yama.rst | 74 | ||||
| -rw-r--r-- | Documentation/admin-guide/LSM/index.rst | 1 |
2 files changed, 75 insertions, 0 deletions
diff --git a/Documentation/admin-guide/LSM/Yama.rst b/Documentation/admin-guide/LSM/Yama.rst new file mode 100644 index 000000000000..13468ea696b7 --- /dev/null +++ b/Documentation/admin-guide/LSM/Yama.rst @@ -0,0 +1,74 @@ +==== +Yama +==== + +Yama is a Linux Security Module that collects system-wide DAC security +protections that are not handled by the core kernel itself. This is +selectable at build-time with ``CONFIG_SECURITY_YAMA``, and can be controlled +at run-time through sysctls in ``/proc/sys/kernel/yama``: + +ptrace_scope +============ + +As Linux grows in popularity, it will become a larger target for +malware. One particularly troubling weakness of the Linux process +interfaces is that a single user is able to examine the memory and +running state of any of their processes. For example, if one application +(e.g. Pidgin) was compromised, it would be possible for an attacker to +attach to other running processes (e.g. Firefox, SSH sessions, GPG agent, +etc) to extract additional credentials and continue to expand the scope +of their attack without resorting to user-assisted phishing. + +This is not a theoretical problem. SSH session hijacking +(http://www.storm.net.nz/projects/7) and arbitrary code injection +(http://c-skills.blogspot.com/2007/05/injectso.html) attacks already +exist and remain possible if ptrace is allowed to operate as before. +Since ptrace is not commonly used by non-developers and non-admins, system +builders should be allowed the option to disable this debugging system. + +For a solution, some applications use ``prctl(PR_SET_DUMPABLE, ...)`` to +specifically disallow such ptrace attachment (e.g. ssh-agent), but many +do not. A more general solution is to only allow ptrace directly from a +parent to a child process (i.e. direct "gdb EXE" and "strace EXE" still +work), or with ``CAP_SYS_PTRACE`` (i.e. "gdb --pid=PID", and "strace -p PID" +still work as root). + +In mode 1, software that has defined application-specific relationships +between a debugging process and its inferior (crash handlers, etc), +``prctl(PR_SET_PTRACER, pid, ...)`` can be used. An inferior can declare which +other process (and its descendants) are allowed to call ``PTRACE_ATTACH`` +against it. Only one such declared debugging process can exists for +each inferior at a time. For example, this is used by KDE, Chromium, and +Firefox's crash handlers, and by Wine for allowing only Wine processes +to ptrace each other. If a process wishes to entirely disable these ptrace +restrictions, it can call ``prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...)`` +so that any otherwise allowed process (even those in external pid namespaces) +may attach. + +The sysctl settings (writable only with ``CAP_SYS_PTRACE``) are: + +0 - classic ptrace permissions: + a process can ``PTRACE_ATTACH`` to any other + process running under the same uid, as long as it is dumpable (i.e. + did not transition uids, start privileged, or have called + ``prctl(PR_SET_DUMPABLE...)`` already). Similarly, ``PTRACE_TRACEME`` is + unchanged. + +1 - restricted ptrace: + a process must have a predefined relationship + with the inferior it wants to call ``PTRACE_ATTACH`` on. By default, + this relationship is that of only its descendants when the above + classic criteria is also met. To change the relationship, an + inferior can call ``prctl(PR_SET_PTRACER, debugger, ...)`` to declare + an allowed debugger PID to call ``PTRACE_ATTACH`` on the inferior. + Using ``PTRACE_TRACEME`` is unchanged. + +2 - admin-only attach: + only processes with ``CAP_SYS_PTRACE`` may use ptrace + with ``PTRACE_ATTACH``, or through children calling ``PTRACE_TRACEME``. + +3 - no attach: + no processes may use ptrace with ``PTRACE_ATTACH`` nor via + ``PTRACE_TRACEME``. Once set, this sysctl value cannot be changed. + +The original children-only logic was based on the restrictions in grsecurity. diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst index 6aa4e0dc588b..e5ba2c69b8ef 100644 --- a/Documentation/admin-guide/LSM/index.rst +++ b/Documentation/admin-guide/LSM/index.rst @@ -36,3 +36,4 @@ the one "major" module (e.g. SELinux) if there is one configured. apparmor SELinux tomoyo + Yama |