Yama is a Linux Security Module that collects a number of 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:

- protected_sticky_symlinks
- protected_nonaccess_hardlinks
- ptrace_scope

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protected_sticky_symlinks:

A long-standing class of security issues is the symlink-based
time-of-check-time-of-use race, most commonly seen in world-writable
directories like /tmp. The common method of exploitation of this flaw
is to cross privilege boundaries when following a given symlink (i.e. a
root process follows a symlink belonging to another user).  For a likely
incomplete list of hundreds of examples across the years, please see:
http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp

When set to "0", symlink following behavior is unrestricted.

When set to "1" symlinks are permitted to be followed only when outside
a sticky world-writable directory, or when the uid of the symlink and
follower match, or when the directory owner matches the symlink's owner.

This protection is based on the restrictions in Openwall and grsecurity.

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protected_nonaccess_hardlinks:

Hardlinks can be abused in a similar fashion to symlinks in sticky
world-writable directories, but their weakness is not limited to
just that scenario. For example, if /etc and /home are on the same
partition, a regular user can create a hardlink to /etc/shadow in their
home directory. While it retains the original owner and permissions,
it is possible for privileged programs that are otherwise symlink-safe
to mistakenly access the file through its hardlink. Additionally, a very
minor untraceable quota-bypassing local denial of service is possible by
an attacker exhausting disk space by filling a world-writable directory
with hardlinks.

When set to "0", hardlink creation behavior is unrestricted.

When set to "1", hardlinks cannot be created to files that a given user
would be unable to read and write originally, or are otherwise sensitive.

This protection is based on the restrictions in Openwall and grsecurity.

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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.
PTRACE is not commonly used by non-developers and non-admins, so 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).

0 - classic PTRACE permissions: a process can PTRACE any other process
    running under the same uid, as long as it is dumpable (i.e. did not
    transition uids, start privileged, or have prctl(PR_SET_DUMPABLE...)
    called).

1 - child-only PTRACE: a process can PTRACE only its descendants when
    the above classic criteria is also met.

This protection is based on the restrictions in grsecurity.

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