On a normal Linux system, /proc/$pid/mem is protected by the same kernel permission check as ptrace, and a Linux security module like Yama, the thing that disables ptracing unrelated processes on Ubuntu etc., will also block this tool (which the README mentions). It seems like it's mostly useful for cases where people are blocking the ptrace syscall (like Docker's default syscall filter, maybe?) and not loading an LSM.
Cool demonstration that blocking the ptrace syscall isn't sufficient. By the way, blocking /proc too isn't sufficient either: there's the process_vm_writev and process_vm_readv syscalls that work like writing/reading /proc/$pid/mem. I think it's harder to write a robust tool using only those syscalls, but I wouldn't bet on it being impossible.
If you really want to do syscall filtering to confine an untrusted process (as opposed to reducing attack surface from potential bugs in otherwise-permissible syscalls, which is I think Docker's goal), you need to start from empty and allow syscalls instead of starting from full and blocking them. Alternatively, maybe just run the untrusted code as another user account or something.
It’s a shot in the dark, but I’d guess this is designed to work with programs that neuter themselves or commit suicide if you ptrace them, as is common for mildly sophisticated malware or (less often) CTF challenges.
a) the program has to actively run its check while being ptraced to notice. If you attach to the program (which pauses it), inject your code, run your code, and then detach, the program's own code will not notice it's being ptraced, no?
b) if you want to run the program's own code while being ptraced, can't you just stop on the ptrace syscall and lie about its result? I think `strace -e inject=ptrace:errno=0` will avoid the program in that wiki page from realizing it's being traced. (You can use seccomp-bpf for this if you don't want to take the performance overhead of stopping on every syscall.)
The readme states that the (primary) shell code is written at the EIP location, so no return hijacking is required.
It looks like the gVisor approach for containers (reimplementing parts of the kernel syscalls in userspace and disallowing the rest)
Also, would this work on systems with ASLR?
Could malicious code be obfuscated, transformed, and then subsequently injected into a process like explorer with this dlinject.py? What are our limitations here?
Yes, there is no simple way to do this. Injecting arbitrary code into a process is always finicky, and this project is even more so because it eschews the API used for doing stuff like this (ptrace, which is still on the level of registers and memory addresses) and going through other kernel interfaces to try to perform similar things.
> Also, would this work on systems with ASLR?
It does, because it defeats ASLR by looking at the process’s memory mappings and finding libc in that (which it uses for dlopen).
> Could malicious code be obfuscated, transformed, and then subsequently injected into a process like explorer with this dlinject.py? What are our limitations here?
None, it can inject pretty much anything. The only limitation is security protections that the OS provides to stop you from touching other processes in this way.
So we need to be able to change the instruction pointer of the process and coax it into calling dlopen(). The functionality available to you is:
- You can use ptrace to stop the process, read and write the saved registers, read and write its memory, resume the process, or resume the process until the next instruction (single step) or next syscall. It's the API used by debuggers. If you run `print dlopen("file.so")` from GDB, this is exactly what GDB will do: it'll use ptrace to make up a stack frame to call dlopen with the arguments you specify and then hit a breakpoint, and GDB will print the result. Another good example of using this to inject code for non-malicious purposes is https://github.com/nelhage/reptyr , which lets you tell another process to switch terminals (e.g., move it into a screen/tmux session).
- You can use /proc/$pid/mem or the process_vm_writev syscall to modify the stack but not the instruction pointer, because the stack is in memory. That puts you effectively in the same position as an exploit writer: you have to create a stack frame that returns to custom code somehow, or uses return-oriented programming to call the function you want to call. Then when the program returns from its current stack frame, it'll call your shellcode or ROP gadget.
- You can use /proc/$pid/mem or the process_vm_writev syscall to modify the code at the instruction pointer, even though you can't modify the instruction pointer itself. That's what this program does - it backs up a segment of instructions about to be executed, overwrites it with shellcode, and restores the instructions once the shellcode is done.
ptrace, /proc/$pid/mem, and process_vm_readv/writev have permission checks. On most systems, you have to be running as the same user as the program you're trying to mess with. On systems including the "Yama" Linux security module - notably including Ubuntu - you also have to be an ancestor process of the process you're trying to attach to, which is a security measure intended to make it harder for malware to steal secrets out of other programs you're running, without preventing debuggers etc. from working. See https://www.kernel.org/doc/Documentation/security/Yama.txt for details.
(Again, this is the same interface as a debugger uses. If you're not prevented from debugging your own programs, yes, you can inject malicious code into them without limitation - and it's generally easier to use a debugger than to write your own custom injector to do it.)
