66 lines
3.2 KiB
Plaintext
66 lines
3.2 KiB
Plaintext
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:
|
|
|
|
- ptrace_scope
|
|
|
|
==============================================================
|
|
|
|
ptrace_scope:
|
|
|
|
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)
|
|
may attach.
|
|
|
|
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.
|
|
|
|
==============================================================
|