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-Yama is a Linux Security Module that collects a number of system-wide DAC
-security protections that are not handled by the core kernel itself. To
-select it at boot time, specify "security=yama" (though this will disable
-any other LSM).
-Yama is controlled through sysctl in /proc/sys/kernel/yama:
-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 descendents) 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)
-These restrictions do not change how ptrace via PTRACE_TRACEME operates.
-The sysctl settings 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).
-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.
-2 - admin-only attach: only processes with CAP_SYS_PTRACE may use ptrace
- with PTRACE_ATTACH.
-3 - no attach: no processes may use ptrace with PTRACE_ATTACH. Once set,
- this sysctl cannot be changed to a lower value.
-The original children-only logic was based on the restrictions in grsecurity.