1 ==== 2 Yama 3 ==== 4 5 Yama is a Linux Security Module that collects system-wide DAC security 6 protections that are not handled by the core kernel itself. This is 7 selectable at build-time with ``CONFIG_SECURITY_YAMA``, and can be controlled 8 at run-time through sysctls in ``/proc/sys/kernel/yama``: 9 10 ptrace_scope 11 ============ 12 13 As Linux grows in popularity, it will become a larger target for 14 malware. One particularly troubling weakness of the Linux process 15 interfaces is that a single user is able to examine the memory and 16 running state of any of their processes. For example, if one application 17 (e.g. Pidgin) was compromised, it would be possible for an attacker to 18 attach to other running processes (e.g. Firefox, SSH sessions, GPG agent, 19 etc) to extract additional credentials and continue to expand the scope 20 of their attack without resorting to user-assisted phishing. 21 22 This is not a theoretical problem. `SSH session hijacking 23 <https://www.blackhat.com/presentations/bh-usa-05/bh-us-05-boileau.pdf>`_ 24 and `arbitrary code injection 25 <https://c-skills.blogspot.com/2007/05/injectso.html>`_ attacks already 26 exist and remain possible if ptrace is allowed to operate as before. 27 Since ptrace is not commonly used by non-developers and non-admins, system 28 builders should be allowed the option to disable this debugging system. 29 30 For a solution, some applications use ``prctl(PR_SET_DUMPABLE, ...)`` to 31 specifically disallow such ptrace attachment (e.g. ssh-agent), but many 32 do not. A more general solution is to only allow ptrace directly from a 33 parent to a child process (i.e. direct "gdb EXE" and "strace EXE" still 34 work), or with ``CAP_SYS_PTRACE`` (i.e. "gdb --pid=PID", and "strace -p PID" 35 still work as root). 36 37 In mode 1, software that has defined application-specific relationships 38 between a debugging process and its inferior (crash handlers, etc), 39 ``prctl(PR_SET_PTRACER, pid, ...)`` can be used. An inferior can declare which 40 other process (and its descendants) are allowed to call ``PTRACE_ATTACH`` 41 against it. Only one such declared debugging process can exists for 42 each inferior at a time. For example, this is used by KDE, Chromium, and 43 Firefox's crash handlers, and by Wine for allowing only Wine processes 44 to ptrace each other. If a process wishes to entirely disable these ptrace 45 restrictions, it can call ``prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...)`` 46 so that any otherwise allowed process (even those in external pid namespaces) 47 may attach. 48 49 The sysctl settings (writable only with ``CAP_SYS_PTRACE``) are: 50 51 0 - classic ptrace permissions: 52 a process can ``PTRACE_ATTACH`` to any other 53 process running under the same uid, as long as it is dumpable (i.e. 54 did not transition uids, start privileged, or have called 55 ``prctl(PR_SET_DUMPABLE...)`` already). Similarly, ``PTRACE_TRACEME`` is 56 unchanged. 57 58 1 - restricted ptrace: 59 a process must have a predefined relationship 60 with the inferior it wants to call ``PTRACE_ATTACH`` on. By default, 61 this relationship is that of only its descendants when the above 62 classic criteria is also met. To change the relationship, an 63 inferior can call ``prctl(PR_SET_PTRACER, debugger, ...)`` to declare 64 an allowed debugger PID to call ``PTRACE_ATTACH`` on the inferior. 65 Using ``PTRACE_TRACEME`` is unchanged. 66 67 2 - admin-only attach: 68 only processes with ``CAP_SYS_PTRACE`` may use ptrace, either with 69 ``PTRACE_ATTACH`` or through children calling ``PTRACE_TRACEME``. 70 71 3 - no attach: 72 no processes may use ptrace with ``PTRACE_ATTACH`` nor via 73 ``PTRACE_TRACEME``. Once set, this sysctl value cannot be changed. 74 75 The original children-only logic was based on the restrictions in grsecurity.
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