Operating System #38 How to Prevent Buffer Overflow Attacks

Operating System #38 How to Prevent Buffer Overflow Attacks

01:30 Non-executable stack:
• Mark the stack pages as non-executable

Non Executable Stack Implementations
• In Intel processors, NX bit present to mark stack as non-executable.
• Works for most programs
• Does not work for some programs that NEED to execute from the stack.
– Eg. Linux signal delivery.

04:13 Return to Libc Attack
10:50 Limitation of ret2libc: “Difficult to execute arbitrary code”

Return Oriented Programming Attacks
• Discovered by Hovav Shacham of Stanford University
• Allows arbitrary computation without code injection
– thus can be used with non executable stacks

16:23 Other Precautions for buffer overflows
• Use a programming language that automatically check array bounds
– Example java
• Use securer libraries. For example C11 annex K, gets_s, strcpy_s, strncpy_s, etc.
(_s is for secure)

Canaries:
Known (pseudo random) values placed on stack to monitor buffer overflows.
• A change in the value of the canary indicates a buffer overflow.
• Implemented in gcc by default.
• Evaded if canary is known

Bounds Checking
• Check accesses to each buffer so that it cannot be beyond the bounds
• In C and C++, bound checking performed at pointer calculation time or dereference time.
• Requires run-time bound information for each allocated block.

Address Space Randomization
• Attackers need to know specific locations in the code.
– For instance, where the stack begins
– Where functions are placed in memory, etc.
• Address space layout randomization (ASLR) makes this difficult by randomizing the address space layout of the process