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