[3/3] Security: Document prctl(PR_{GET,SET}_NETWORK). (v3)

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+Purpose
+-------
+
+Daniel Bernstein has observed [1] that security-conscious userland processes
+may benefit from the ability to irrevocably remove their ability to create,
+bind, connect to, or send messages except in the case of previously connected
+sockets or AF_UNIX filesystem sockets.
+
+This facility is particularly attractive to security platforms like OLPC
+Bitfrost [2] and to isolation programs like Rainbow [3] and Plash [4] because:
+
+  * it integrates well with standard techniques for writing privilege-separated
+    Unix programs
+
+  * it integrates well with the need to perform limited socket I/O, e.g., when
+    running X clients
+
+  * it's available to unprivileged programs
+
+  * it's a discretionary feature available to all of distributors,
+    administrators, authors, and users
+
+  * its effect is entirely local, rather than global (like netfilter)
+
+  * it's simple enough to have some hope of being used correctly
+
+Implementation
+--------------
+
+After considering implementations based on the Linux Security Module (LSM)
+framework, on SELinux, on network namespaces (CLONE_NEWNET), on direct
+modification of the kernel syscall and task_struct APIs and after seeking
+advice from members of LKML, we came to the conclusion that the best way to
+implement this feature was to extend the prctl() framework with a new pair of
+options named PR_{GET,SET}_NETWORK and to write an LSM to implement the
+resulting PR_NETWORK_OFF semantics. These options cause prctl() to read or
+modify "current->network".
+
+Semantics
+---------
+
+current->network is a flags field which is preserved across all variants of
+fork() and exec().
+
+Writes which attempt to clear bits in current->network return -EPERM.
+
+The default value for current->network is named PR_NETWORK_ON and is defined
+to be 0.
+
+Presently, only one flag is defined: PR_NETWORK_OFF.
+
+More flags may be defined in the future if they become needed.
+
+Attempts to set undefined flags result in -EINVAL.
+
+When PR_NETWORK_OFF is set, implementations of syscalls which may be used by
+the current process to perform autonomous networking will return -EPERM. For
+example, calls to socket(), bind(), connect(), sendmsg(), and ptrace() will
+return -EPERM except for cases we are manipulating an AF_UNIX socket or, in the
+case of sendmsg(), unless we are manipulating a previously connected socket,
+i.e. one with
+
+  msg.msg_name == NULL && msg.msg_namelen == 0
+
+or, in the case of ptrace(), unless we are ptracing() a process which has all
+of our own networking restriction flags set.
+
+References
+----------
+
+[1]: http://cr.yp.to/unix/disablenetwork.html
+[2]: http://wiki.laptop.org/go/OLPC_Bitfrost
+[3]: http://wiki.laptop.org/go/Rainbow
+[4]: http://plash.beasts.org/