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authorKees Cook <keescook@chromium.org>2017-05-13 14:51:47 +0300
committerJonathan Corbet <corbet@lwn.net>2017-05-18 19:33:04 +0300
commit90bb766440f2147486a2acc3e793d7b8348b0c22 (patch)
treed70d83fa1075ef5434f3e049ae2ffa3b5096fae4 /Documentation/admin-guide/LSM/Yama.rst
parent5ea672c752d93ef3cfa9ce3ea1fbf204f7056a33 (diff)
downloadlinux-90bb766440f2147486a2acc3e793d7b8348b0c22.tar.xz
doc: ReSTify Yama.txt
Adjusts for ReST markup and moves under LSM admin guide. Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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+====
+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.