Add goldarn writeup

sploits/goldarn/README.md

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+# Goldarn
+
+Goldarn is an online stack-based language interpreter. The service allows users to access a REPL and use any command available in language.
+
+Flags were stored in content of randomly named files.
+
+Service is written in Rust and allows reading and writing of any file on the system. Also a C-wrapper is used to apply seccomp and to change uid and gid to `1337 + rand[0, 256)`. The [gotty](https://github.com/yudai/gotty) binary was used to interact with REPL via websockets.
+
+## Home overriding
+
+### Gotty config exploit
+
+Service is using gotty v1.0.1 to run `cargo run` [command](../../services/goldarn/run_gotty.sh) on each connection. The gotty command was run by a user with uid 1337 that had an existing home directory created in [Dockerfile](../../services/goldarn/Dockerfile). The cargo command was run by a user with uid `1337 + rand[0, 256)` which that with 1/256 probability we can write and read files in 1337 user home directory. Lucky for us gotty supports [configuration files](https://github.com/yudai/gotty/blob/v1.0.1/.gotty) in home directory. One interesting option is `permit_arguments` which allows user to pass additional arguments to underlying binary (in our case it's cargo). We can use this to overwrite path to Cargo.toml via `--manifest-path` and run binary from Rust project located in different directory. The last step is to find directory we can write to and craft a complete Rust project structure in it (Cargo.toml and one source file). `/tmp` directory can be used only once - since cargo doesn't allow overriding Cargo.toml file name and service doesn't allow overwriting files, only appending. At this moment we can see that C-wrapper creates temporary directory for each execution and it can be used for explotation. Exploit reads `/proc/self/environ` file to find location of this directory via `LD_LIBRARY_PATH` env variable.
+
+### Cargo config exploit (unintended, from [Superflat](https://ctftime.org/team/274071/) team)
+
+```
+We (Superflat) used a different exploit for goldarn: checker setuids to 1337+random_byte, so its possible for uid to stay 1337 (ctf user). This user can write to $CARGO_HOME/config.toml and use it to overwrite runner for the default triple
+```
+
+### Fix
+
+Change `setuid(1337 + rand[0, 256))` to `setuid(1337 + rand[1, 256))` so overriding of home directory become impossible.