Add security tips about file and paths

doc/development/shell_commands.md

 # Guidelines for shell commands in the GitLab codebase
 
+This document contains guidelines for working with processes and files in the GitLab codebase.
+These guidelines are meant to make your code more reliable _and_ secure.
+
 ## References
 
 - [Google Ruby Security Reviewer's Guide](https://code.google.com/p/ruby-security/wiki/Guide)
@@ -109,3 +112,63 @@ logs = IO.popen(%W(git log), chdir: repo_dir).read
 ```
 
 Note that unlike `Gitlab::Popen.popen`, `IO.popen` does not capture standard error.
+
+## Avoid user input at the start of path strings
+
+Various methods for opening and reading files in Ruby can be used to read the
+standard output of a process instead of a file.  The following two commands do
+roughly the same:
+
+```
+`touch /tmp/pawned-by-backticks`
+File.read('|touch /tmp/pawned-by-file-read')
+```
+
+The key is to open a 'file' whose name starts with a `|`.
+Affected methods include Kernel#open, File::read, File::open, IO::open and IO::read.
+
+You can protect against this behavior of 'open' and 'read' by ensuring that an
+attacker cannot control the start of the filename string you are opening.  For
+instance, the following is sufficient to protect against accidentally starting
+a shell command with `|`:
+
+```
+# we assume repo_path is not controlled by the attacker (user)
+path = File.join(repo_path, user_input)
+# path cannot start with '|' now.
+File.read(path)
+```
+
+## Guard against path traversal
+
+Path traversal is a security where the program (GitLab) tries to restrict user
+access to a certain directory on disk, but the user manages to open a file
+outside that directory by taking advantage of the `../` path notation.
+
+```
+# Suppose the user gave us a path and they are trying to trick us
+user_input = '../other-repo.git/other-file'
+
+# We look up the repo path somewhere
+repo_path = 'repositories/user-repo.git'
+
+# The intention of the code below is to open a file under repo_path, but
+# because the user used '..' she can 'break out' into
+# 'repositories/other-repo.git'
+full_path = File.join(repo_path, user_input)
+File.open(full_path) do # Oops!
+```
+
+A good way to protect against this is to compare the full path with its
+'absolute path' according to Ruby's `File.absolute_path`.
+
+```
+full_path = File.join(repo_path, user_input)
+if full_path != File.absolute_path(full_path)
+  raise "Invalid path: #{full_path.inspect}"
+end
+
+File.open(full_path) do # Etc.
+```
+
+A check like this could have avoided CVE-2013-4583.