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Chet Ramey
55 lines
2.7 KiB
Plaintext
55 lines
2.7 KiB
Plaintext
6.10 The Restricted Shell
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=========================
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If Bash is started with the name ‘rbash’, or the ‘--restricted’ or ‘-r’
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option is supplied at invocation, the shell becomes RESTRICTED. A
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restricted shell is used to set up an environment more controlled than
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the standard shell. A restricted shell behaves identically to ‘bash’
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with the exception that the following are disallowed or not performed:
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• Changing directories with the ‘cd’ builtin.
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• Setting or unsetting the values of the ‘SHELL’, ‘PATH’, ‘HISTFILE’,
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‘ENV’, or ‘BASH_ENV’ variables.
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• Specifying command names containing slashes.
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• Specifying a filename containing a slash as an argument to the ‘.’
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builtin command.
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• Using the ‘-p’ option to the ‘.’ builtin command to specify a
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search path.
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• Specifying a filename containing a slash as an argument to the
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‘history’ builtin command.
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• Specifying a filename containing a slash as an argument to the ‘-p’
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option to the ‘hash’ builtin command.
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• Importing function definitions from the shell environment at
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startup.
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• Parsing the value of ‘SHELLOPTS’ from the shell environment at
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startup.
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• Redirecting output using the ‘>’, ‘>|’, ‘<>’, ‘>&’, ‘&>’, and ‘>>’
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redirection operators.
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• Using the ‘exec’ builtin to replace the shell with another command.
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• Adding or deleting builtin commands with the ‘-f’ and ‘-d’ options
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to the ‘enable’ builtin.
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• Using the ‘enable’ builtin command to enable disabled shell
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builtins.
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• Specifying the ‘-p’ option to the ‘command’ builtin.
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• Turning off restricted mode with ‘set +r’ or ‘shopt -u
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restricted_shell’.
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These restrictions are enforced after any startup files are read.
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When a command that is found to be a shell script is executed (*note
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Shell Scripts::), ‘rbash’ turns off any restrictions in the shell
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spawned to execute the script.
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The restricted shell mode is only one component of a useful restricted
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environment. It should be accompanied by setting ‘PATH’ to a value that
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allows execution of only a few verified commands (commands that allow
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shell escapes are particularly vulnerable), changing the current
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directory to a non-writable directory other than ‘$HOME’ after login,
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not allowing the restricted shell to execute shell scripts, and cleaning
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the environment of variables that cause some commands to modify their
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behavior (e.g., ‘VISUAL’ or ‘PAGER’).
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Modern systems provide more secure ways to implement a restricted
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environment, such as ‘jails’, ‘zones’, or ‘containers’.
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