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Documentation / security / Yama.txt

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Based on kernel version 4.3. Page generated on 2015-11-02 12:51 EST.

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