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Documentation / security / Yama.txt

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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	Yama is a Linux Security Module that collects a number of system-wide DAC
2	security protections that are not handled by the core kernel itself. To
3	select it at boot time, specify "security=yama" (though this will disable
4	any other LSM).
6	Yama is controlled through sysctl in /proc/sys/kernel/yama:
8	- ptrace_scope
10	==============================================================
12	ptrace_scope:
14	As Linux grows in popularity, it will become a larger target for
15	malware. One particularly troubling weakness of the Linux process
16	interfaces is that a single user is able to examine the memory and
17	running state of any of their processes. For example, if one application
18	(e.g. Pidgin) was compromised, it would be possible for an attacker to
19	attach to other running processes (e.g. Firefox, SSH sessions, GPG agent,
20	etc) to extract additional credentials and continue to expand the scope
21	of their attack without resorting to user-assisted phishing.
23	This is not a theoretical problem. SSH session hijacking
24	(http://www.storm.net.nz/projects/7) and arbitrary code injection
25	(http://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.
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).
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 descendents) 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.
49	The sysctl settings (writable only with CAP_SYS_PTRACE) are:
51	0 - classic ptrace permissions: a process can PTRACE_ATTACH to any other
52	    process running under the same uid, as long as it is dumpable (i.e.
53	    did not transition uids, start privileged, or have called
54	    prctl(PR_SET_DUMPABLE...) already). Similarly, PTRACE_TRACEME is
55	    unchanged.
57	1 - restricted ptrace: a process must have a predefined relationship
58	    with the inferior it wants to call PTRACE_ATTACH on. By default,
59	    this relationship is that of only its descendants when the above
60	    classic criteria is also met. To change the relationship, an
61	    inferior can call prctl(PR_SET_PTRACER, debugger, ...) to declare
62	    an allowed debugger PID to call PTRACE_ATTACH on the inferior.
63	    Using PTRACE_TRACEME is unchanged.
65	2 - admin-only attach: only processes with CAP_SYS_PTRACE may use ptrace
66	    with PTRACE_ATTACH, or through children calling PTRACE_TRACEME.
68	3 - no attach: no processes may use ptrace with PTRACE_ATTACH nor via
69	    PTRACE_TRACEME. Once set, this sysctl value cannot be changed.
71	The original children-only logic was based on the restrictions in grsecurity.
73	==============================================================
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