Using untrusted data
Posted by BladeSabre (BladeSabre), 31 January 2006I'm looking at Tcl safe interpreters. The manual tells me that "the most important thing in guaranteeing safety is to ensure that information passed from the slave to the master is never evaluated or substituted in the master". I understand why that is, but I'm not quite sure what I am allowed to do with it. The implication is that I can do anything that doesn't involve evaluating or substituting, but I'm not clear on what falls into that category.
Searching the Web has just given me the same phrases many times over. So I was hoping someone could explain this to me.
Posted by admin (Graham Ellis), 31 January 2006Tcl is an unusual language in that it uses the same internal logic to handle both data and program. Data held as a list has the same syntax as program command parameters, and so you need to take care that there's no crossover. Beyond that (and working from memory because I'm out of the office and away from my books) there's no big problems with doing things with data passed in to safe interpretters ... "tricks" like trying to open files in directories way off the tree shouldn't work, for example.
I feel I've just written a muddy answer; would you like me to have a further look towards the end of the week when I'll be able to write something a bit more sane?
Posted by BladeSabre (BladeSabre), 31 January 2006Hi, thanks for your quick response.
What I'm worried about is data passed out of safe interpreters. (The line I quoted from the manual refers to that.) Either passing arguments using aliases, or if the master calls functions in the slave and receives their return values- I think it has the same effect, from this point of view. Either way, I have a piece of untrusted data in the master, and I'm not sure what is and isn't OK to do with it.
Examples of things I know not to do, and why (pretending that p is a "bad thing to happen"):
Apparently the top pair execute the string as a command, which I understand. The bottom pair go through the substitution process twice. That's largely what confuses me- I'm not sure if anything else will have that effect.
I think these are OK, because they only substitute once:
But I'm not sure. I really don't know how to tell what's OK and what's not.
If you have more time (and books) in a few days, I would be glad of more information.
Posted by BladeSabre (BladeSabre), 15 February 2006Hi. I wondered if you were going to come back to this.
Posted by admin (Graham Ellis), 16 February 2006oops - I had overlooked it. Dashing now but it's back on my list
Posted by BladeSabre (BladeSabre), 17 April 2006Well, it's me again. I found a website that has been very helpful (link), and I think I've got it now, though I'm not confident.
This program is a game, and "profiles.txt" contains some untrusted player-written Tcl that's supposed to follow a specific structure. It's not intended to be executable on its own, but there's a few helper procedures that the master interpreter adds. The plan is that the code inside the slave (player-written and what's given by the master) holds a set of status variables, and updates them when the master calls specific procedures. The slave is also supposed to compute and return integer values (or error strings) on request.
The idea is that what I've written here should contain all the dangerous stuff, so I can call the 4 (well, 6) procedures at the bottom, from the rest of the program, without worrying about it. However, there's still a good chance I've done something silly. If you (or anyone else here) would be willing to give me some feedback on my code, I would greatly appreciate that.
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