EasySandbox

An easy way to sandbox untrusted C and C++ programs on Linux. Allows them only to allocate memory (up to a fixed amount), read and write via stdin, stdout, and stderr, and exit. Sandboxing is done using SECCOMP.

The intended use is being able to safely execute student code submissions for the CloudCoder programming exercise system, although it could be useful in other contexts.

You can run

make runtests

to run the test programs. If you see "All tests passed!", then EasySandbox is working on your system.

EasySandbox is distributed under the MIT license.

If you have questions about EasySandbox, send me an email. If you have improvements that you would like to share, send me a pull request on GitHub.

Using EasySandbox

Run the make command to build the EasySandbox shared library.

Run the program you want to sandbox, using the LD_PRELOAD environment variable to load the EasySandbox shared library before the untrusted executable is executed:

LD_PRELOAD=/path/to/EasySandbox.so ./untrustedExe

EasySandbox defines its own implementation of malloc and free, to ensure that the program will not need to call sbrk or mmap to allocate memory while in SECCOMP mode. The heap is a fixed size, and cannot grow while the program is running. You can control the size of the heap by setting the EASYSANDBOX_HEAPSIZE environment variable to the size of the heap in bytes. The default heap size is 8MB.

Note: EasySandbox uses __libc_start_main to hook into the startup process. If the untrusted executable defines its own entry point (rather than the normal Linux/glibc one), it could execute untrusted code. In my intended application (compiling and executing student code submissions), I control the compilation process, and I believe that as long as gcc/g++ is invoked without the -nostdlib option, any attempt by the untrusted code to define an entry point (_start function) will result in a linker error, because the name _start will conflict with the real _start function defined in crt1.o.

Important: A program that calls dlopen might be able to bypass EasySandbox's __libc_start_main: see this issue. Make sure that untrusted code is not linked with -ldl.

Limitations

When you execute a program using EasySandbox, it will print the message

<<entering SECCOMP mode>>

followed by a newline character to both stdout and stderr. The reason is that the first call to print to an output stream causes glibc to invoke fstat, which is not permitted when in SECCOMP mode. So, the EasySandbox shared library must print some output to stdout and stderr before entering SECCOMP mode in order for these streams to be usable. It is fairly easy to filter out this output as a post-processing step.

Similarly, reading from stdin also triggers a call to fstat. The EasySandbox shared library works around this by putting the stdin file descriptor into nonblocking mode, attempting to read a single character using the fgetc function, and then using ungetc function to put the character back if one was read. This should not cause any problems for programs that use C library functions to read from stdin, but programs that use the read system call to read from the stdin file descriptor may not be able to read the first byte of input.

The EasySandbox shared library implements its own exit function, because glibc's invokes the exit_group system call, which is not allowed by SECCOMP. The behavior of this custom exit function attempts to emulate glibc's: it runs atexit functions, which includes destructors for static C++ objects.

EasySandbox is not intended to be used for multithreaded programs. SECCOMP will surely kill any process that attempts to create an additional thread, since creating a thread would require an invocation of the clone system call, which isn't allowed by SECCOMP.

EasySandbox is designed to work with glibc, and may or may not work with other libc variants. It is entirely possible that future changes to glibc could break EasySandbox.