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