Diffstat (limited to 'Documentation/security/Yama.txt')
1 files changed, 65 insertions, 0 deletions
diff --git a/Documentation/security/Yama.txt b/Documentation/security/Yama.txt
new file mode 100644
@@ -0,0 +1,65 @@
+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).
+For 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)
+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.
+The original children-only logic was based on the restrictions in grsecurity.