exercise-2

Exercise: Adapt a C Program to CHERI C

This exercise presents an example C program that includes capability-related issues that might appear as bugs in software initially developed for non-CHERI architectures. The example C program is cat(1) from CheriBSD (and hence FreeBSD) modified to introduce the issues that we want to investigate.

1. Compile the program for CheriABI and the hybrid ABI, and save the compiler output to analyse it later

   make
   

2. Run the cat program for both ABIs with an arbitrary file, e.g.:

   ./cat-baseline /etc/resolv.conf
   

   ./cat-cheri /etc/resolv.conf
   

3. Analyse the ambiguous provenance issue reported by the compiler and try to fix it

   • The write(2) system call expects a valid pointer in its second argument

   • Run the program with CHERI GDB and a breakpoint set for the write symbol. Once CHERI GDB hits the breakpoint, you can use the backtrace GDB command to see what function calls write(2). You can do this by entering GDB: gdb ./cat-cheri Then once inside set the breakpoint using: break methods.c:70 Finally run our vulnerable code: run /etc/resolv.conf

   • What is the value of the buf argument and why?

   • After fixing the issue, repeat the CHERI GDB session to see what value the second write(2) system call argument has now

4. Run the cat program in the verbose mode (-n) for both ABIs with an arbitrary file, e.g.:

   ./cat-baseline -n /etc/resolv.conf
   

   ./cat-cheri -n /etc/resolv.conf
   

5. Analyse the loss of provenance issue reported by the compiler and try to fix it

   • The getc() is a macro that expects a valid FILE pointer

     The macro accesses fields of a FILE object.

   • Run the program with GDB to repeat the crash. Again, use the backtrace GDB command to see what is the crash context

   • What is the value of the fp argument and why?