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Kees Cook2d514482011-12-21 12:17:04 -08001Yama is a Linux Security Module that collects a number of system-wide DAC
2security protections that are not handled by the core kernel itself. To
3select it at boot time, specify "security=yama" (though this will disable
4any other LSM).
5
6Yama is controlled through sysctl in /proc/sys/kernel/yama:
7
8- ptrace_scope
9
10==============================================================
11
12ptrace_scope:
13
14As Linux grows in popularity, it will become a larger target for
15malware. One particularly troubling weakness of the Linux process
16interfaces is that a single user is able to examine the memory and
17running 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
19attach to other running processes (e.g. Firefox, SSH sessions, GPG agent,
20etc) to extract additional credentials and continue to expand the scope
21of their attack without resorting to user-assisted phishing.
22
23This 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
26exist and remain possible if ptrace is allowed to operate as before.
27Since ptrace is not commonly used by non-developers and non-admins, system
28builders should be allowed the option to disable this debugging system.
29
30For a solution, some applications use prctl(PR_SET_DUMPABLE, ...) to
31specifically disallow such ptrace attachment (e.g. ssh-agent), but many
32do not. A more general solution is to only allow ptrace directly from a
33parent to a child process (i.e. direct "gdb EXE" and "strace EXE" still
34work), or with CAP_SYS_PTRACE (i.e. "gdb --pid=PID", and "strace -p PID"
35still work as root).
36
Kees Cook389da252012-04-16 11:56:45 -070037In mode 1, software that has defined application-specific relationships
Kees Cook2d514482011-12-21 12:17:04 -080038between a debugging process and its inferior (crash handlers, etc),
39prctl(PR_SET_PTRACER, pid, ...) can be used. An inferior can declare which
40other process (and its descendents) are allowed to call PTRACE_ATTACH
41against it. Only one such declared debugging process can exists for
42each inferior at a time. For example, this is used by KDE, Chromium, and
43Firefox's crash handlers, and by Wine for allowing only Wine processes
Kees Cookbf061892012-02-14 16:48:09 -080044to ptrace each other. If a process wishes to entirely disable these ptrace
45restrictions, it can call prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...)
46so that any otherwise allowed process (even those in external pid namespaces)
47may attach.
48
Kees Cook9d8dad72012-08-09 19:01:26 -070049The sysctl settings (writable only with CAP_SYS_PTRACE) are:
Kees Cook2d514482011-12-21 12:17:04 -080050
510 - 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
Kees Cook9d8dad72012-08-09 19:01:26 -070054 prctl(PR_SET_DUMPABLE...) already). Similarly, PTRACE_TRACEME is
55 unchanged.
Kees Cook2d514482011-12-21 12:17:04 -080056
571 - 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.
Kees Cook9d8dad72012-08-09 19:01:26 -070063 Using PTRACE_TRACEME is unchanged.
Kees Cook2d514482011-12-21 12:17:04 -080064
Kees Cook389da252012-04-16 11:56:45 -0700652 - admin-only attach: only processes with CAP_SYS_PTRACE may use ptrace
Kees Cook9d8dad72012-08-09 19:01:26 -070066 with PTRACE_ATTACH, or through children calling PTRACE_TRACEME.
Kees Cook389da252012-04-16 11:56:45 -070067
Kees Cook9d8dad72012-08-09 19:01:26 -0700683 - no attach: no processes may use ptrace with PTRACE_ATTACH nor via
69 PTRACE_TRACEME. Once set, this sysctl value cannot be changed.
Kees Cook389da252012-04-16 11:56:45 -070070
Kees Cook2d514482011-12-21 12:17:04 -080071The original children-only logic was based on the restrictions in grsecurity.
72
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