[PATCH] Security: Document RLIMIT_NETWORK.

[Michael Stone <michael@laptop.org>]

Signed-off-by: Michael Stone <michael@laptop.org>
---
  Documentation/rlimit_network.txt |   55 ++++++++++++++++++++++++++++++++++++++
  1 files changed, 55 insertions(+), 0 deletions(-)
  create mode 100644 Documentation/rlimit_network.txt

diff --git a/Documentation/rlimit_network.txt b/Documentation/rlimit_network.txt
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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 in particular, on network namespaces (CLONE_NEWNET), and
+on direct modification of the kernel syscall and task_struct APIs, we came to
+the conclusion that the best way to implement this feature was to extend the
+resource limits framework with a new RLIMIT_NETWORK field and to modify the
+implementations of the relevant socket calls to return -EPERM when
+
+  current->signal->rlim[RLIMIT_NETWORK].rlim_cur == 0
+
+unless we are manipulating an AF_UNIX socket whose name does not begin with \0
+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
+
+Finally, in response to criticism from Alan Cox, we insert a similar access
+check into __ptrace_may_access() to prevent processes which have dropped their
+networking privileges from performing network I/O by ptracing other processes.
+
+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/
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1.5.6.5