Stack Alignment

A minor issue

A small issue you may get when pwning on 64-bit systems is that your exploit works perfectly locally but fails remotely - or even fails when you try to use the provided LIBC version rather than your local one. This arises due to something called stack alignment.

Essentially the x86-64 ABI (application binary interface) guarantees 16-byte alignment on a call instruction. LIBC takes advantage of this and uses SSE data transfer instructions to optimise execution; system in particular utilises instructions such as movaps.

That means that if the stack is not 16-byte aligned - that is, RSP is not a multiple of 16 - the ROP chain will fail on system.

The fix is simple - in your ROP chain, before the call to system, place a singular ret gadget:

ret = elf.address + 0x2439

rop.raw(0x4)        # first parameter
rop.raw(ret)        # align the stack

This works because it will cause RSP to be popped an additional time, pushing it forward by 8 bytes and aligning it.

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