Re: Updated: [PATCH] hardening: add PROT_FINAL prot flag to mmap/mprotect

[Mikael Pettersson]

Ard Biesheuvel writes:
 > This patch adds support for the PROT_FINAL flag to
 > the mmap() and mprotect() syscalls.
 > 
 > The PROT_FINAL flag indicates that the requested set
 > of protection bits should be final, i.e., it shall
 > not be allowed for a subsequent mprotect call to
 > set protection bits that were not set already.
 > 
 > This is mainly intended for the dynamic linker,
 > which sets up the address space on behalf of
 > dynamic binaries. By using this flag, it can
 > prevent exploited code from remapping read-only
 > executable code or data sections read-write.

I can see why you might think this is a good idea, but I don't
like it for several reasons:

- If .text is mapped non-writable and final, how would a debugger
  (or any ptrace-using monitor-like application) plant a large
  number of breakpoints in a target process? Breakpoint registers
  aren't enough because (a) they're few in number, and (b) not
  all CPUs have them.

- You're proposing to give one component (the dynamic linker/
  loader) absolute power to impose new policies on all
  applications. How would an application that _deliberately_
  does something the new policies don't allow tell the dynamic
  linker or kernel to get out of its way?

This clearly changes the de-facto ABIs, and as such I think
it needs much more detailed analysis than what you've done
here.

At the very least I think this change should be opt-in, but
that would require a kernel option or sysctl, or some config
file for the user-space dynamic linker/loader.
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