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How do you enable Node.JS to spawn a new process that runs inside an SELinux Sandbox?

Posted on 2013-05-17
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Last Modified: 2013-06-01
I am building a Node.JS application that involves redirecting user input into a server-side command.  Of course, that could be catastrophic to security, so I desire to run the child command inside an SELinux sandbox.  (I do not want to run the entire application inside of a sandbox because I want the end users to each have their own workspace on the server.)

For example, for the purpose of demonstration, consider the command `cowsay`.  In order to run a sandboxed cowsay, you need simply `sandbox cowsay`.  Other than the behind-the-scenes security differences, the interface of `sandbox cowsay` should be the same as that of plain `cowsay`.

However, Node.JS responds differently to these two approaches.  Consider the code:

var spawn = require('child_process').spawn;                      
var cmd = spawn("cowsay", ["hello"]); // line A (no sandbox) 
var cmd = spawn("sandbox", ["cowsay", "hello"]); // line B (with sandbox)
cmd.stdout.on("data", function(data){
    console.log("stdout: "+data);
});
cmd.stderr.on("data", function(data){
    console.log("stderr: "+data);
});
cmd.on("exit", function(err){
    console.log("exit: "+err);
});

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Here is the output of the version with line A:
$ node run.js
stdout:  _______ 
< hello >
 ------- 
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

exit: 0

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but here is the output of the version with line B instead:
$ node run.js
exit: 0

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In other words, Node.JS does not seem to read the streams from sandbox's child process.

This behavior is exhibited only when SELinux is in "enforcing" mode.  The sandbox version works fine when SELinux is in "permissive" (non-enforcing) mode.

This test can be performed using any command.  For example, suppose we used the python command interpreter.  What happens here is that the non-sandboxed `python` waits for something to be fed into its stdin from Node.JS, but the sandboxed `python` simply exists with code 0, without waiting.

What needs to happen in order for Node.JS to treat a sandboxed command the same as a non-sandboxed command?
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Question by:sffc
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6 Comments
 
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by:sffc
ID: 39212490
I eventually solved the issue.  I posted my solution here.
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sffc earned 0 total points
ID: 39213929
The issue seems to involve SELinux stopping libuv (the underpinnings of Node) from reading and writing a UNIX stream socket, which of course is required for Node's non-blocking I/O.  One reason this took so long to find was that this error was being hidden from SELinux's audit by dontaudit rules.

We started by doing the following:

# semodule -DB  ## turns off dontaudit rules
# tail -f /var/log/audit/audit.log

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Then in a different terminal, run the application.  A bunch of SELinux audits appear, but most significantly, one that says:

type=AVC msg=audit(1370065734.791:5038): avc:  denied  { read write } for  pid=8887 comm="my_command" path="socket:[1258916]" dev=sockfs ino=1258916 scontext=unconfined_u:unconfined_r:sandbox_t:s0:c98,c807 tcontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 tclass=unix_stream_socket

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To make this into a policy, we did the following:

# grep "type=AVC msg=audit(1370065734.791:5038)" /var/log/audit/audit.log  | audit2allow -M unixsocketsandbox
# semodule -i unixsocketsandbox.pp

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where `unixsocketsandbox` is a unique name for the policy.  The policy (via `cat unixsocketsandbox.te`) reads as follows:

allow sandbox_t unconfined_t:unix_stream_socket { read write };

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After this, the application works as expected, within the limitations of a sandbox.
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