Based on kernel version 4.2. Page generated on 2015-09-09 12:15 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). 5 6 Yama is controlled through sysctl in /proc/sys/kernel/yama: 7 8 - ptrace_scope 9 10 ============================================================== 11 12 ptrace_scope: 13 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. 22 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. 29 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). 36 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 descendants) 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. 48 49 The sysctl settings (writable only with CAP_SYS_PTRACE) are: 50 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. 56 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. 64 65 2 - admin-only attach: only processes with CAP_SYS_PTRACE may use ptrace 66 with PTRACE_ATTACH, or through children calling PTRACE_TRACEME. 67 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. 70 71 The original children-only logic was based on the restrictions in grsecurity. 72 73 ==============================================================