PoC for a shell interpreter sandbox.

[alecthomas]

This uses mvdan/sh/interp to provide a builtin POSIX shell interpreter
whose file operations are (mostly) restricted to a sandbox root. I say
mostly because the builtin shell globbing (eg. echo /etc/*) still
allows listing directories outside the sandbox, though accessing them is
prevented, but also because I'm not certain about those guarantees.

The basics seem to work though there would be quite a bit more work
fleshing out the supported utilities.

Security wise I'm not sure this gives us much, as one of the goals of
this is to allow executables within the sandbox to be executed (eg.
Java's keytool). If this is allowed then the package can basically
execute arbitrary code without restriction, so I don't think there are
many/any security benefits whatsoever.

However it does vastly simplify hooks while simultaneously making them much more flexible, eg.

diff --git a/openjdk.hcl b/openjdk.hcl
index e64c64b..bcb0fca 100644
--- a/openjdk.hcl
+++ b/openjdk.hcl
@@ -9,20 +9,12 @@ strip = 1
 
 // Install root certificate
 on unpack {
-    chmod { mode = 0600 file = "${root}/lib/security/cacerts" }
-    copy { from = "cacerts/g2.pem" to = "${dest}/cacert.pem" }
-    run {
-        cmd = "${root}/bin/keytool"
-        args = [
-            "-importcert",
-            "-noprompt",
-            "-storepass", "changeit",
-            "-keystore", "${root}/lib/security/cacerts",
-            "-alias",  "g2",
-            "-file", "${dest}/cacert.pem"
-        ]
-    }
-    chmod { mode = 0400 file = "${root}/lib/security/cacerts" }
+    exec = <<-EOF
+      chmod 600 lib/security/cacerts
+      cp "source:///cacerts/g2.pem" "${dest}/cacert.pem"
+      bin/keytool -importcert -noprompt -storepass changeit -keystore "lib/security/cacerts" -alias g2 -file "${dest}/cacert.pem"
+      chmod 400 lib/security/cacerts
+    EOF
 }
 
 darwin {

There are also other benefits such as consistent shell support across any
OS, including Windows, no need to rely on particular versions
of bash being present, and some safety guarantees around accidentally
violating the sandbox - eg. a script that accidentally rm's some files.