AP_Scripting: adjust string metatable setup to fix sandbox integrity #27666
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In Lua, strings are the only type that come with a default metatable. The metatable must be shared by all string objects, and it is set to be the `string` library table each time that library is opened. In Ardupilot's scripting engine, the last script to load then has access to the string metatable as the library is opened fresh for each script, as its `string` library will have been set to the metatable. Therefore, if two scripts are loaded, A first and B second, and script B executes e.g. `string.byte = "haha"`, then `string.byte()` and `s:byte()` for script B are broken. Because the metatable is shared, this also breaks `s:byte()` for script A, which violates the integrity of the sandbox. Fix the issue by disabling the metatable setup functionality when the string libary is opened, then manually opening an additional copy of the library (which won't be given to any script) and setting it as the string metatable during intialization. This will break any script that modifies the string metatable for constructive purposes, but such a script could have been broken if it weren't the only script running anyway.
IamPete1
approved these changes
Jul 28, 2024
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In Lua, strings are the only type that come with a default metatable. The metatable must be shared by all string objects, and it is set to be the
stringlibrary table each time that library is opened. In Ardupilot's scripting engine, the last script to load then has access to the string metatable as the library is opened fresh for each script, as itsstringlibrary will have been set to the metatable.Therefore, if two scripts are loaded, A first and B second, and script B executes e.g.
string.byte = "haha", thenstring.byte()ands:byte()for script B are broken. Because the metatable is shared, this also breakss:byte()for script A, which violates the integrity of the sandbox.Fix the issue by disabling the metatable setup functionality when the string libary is opened, then manually opening an additional copy of the library (which won't be given to any script) and setting it as the string metatable during intialization.
This will break any script that modifies the string metatable for constructive purposes, but such a script could have been broken if it weren't the only script running anyway.
Tested on SITL and Cube Orange with my REPL. Saves 24 bytes of flash on Cube Orange, but costs 770 bytes of Lua state RAM, which I do not love.