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Diffstat (limited to 'doc/development/shell_commands.md')
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diff --git a/doc/development/shell_commands.md b/doc/development/shell_commands.md index 23c8365c340..821027f43fa 100644 --- a/doc/development/shell_commands.md +++ b/doc/development/shell_commands.md @@ -1,5 +1,8 @@ # 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) @@ -105,7 +108,72 @@ In other repositories, such as gitlab-shell you can also use `IO.popen`. ```ruby # Safe IO.popen example -logs = IO.popen(%W(git log), chdir: repo_dir).read +logs = IO.popen(%W(git log), chdir: repo_dir) { |p| p.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) +``` + +If you have to use user input a relative path, prefix `./` to the path. + +Prefixing user-supplied paths also offers extra protection against paths +starting with `-` (see the discussion about using `--` above). + +## 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. |