How do you get an x32 Ubuntu? I couldn't find it on the regular download page. I'm not surprised since it is a rather obscure configuration.
I don't run Ubuntu myself, but CONFIG_X86_X32 isn't a new architecture, it's an option for 64 bit kernels. Based on the state of that bug report, it looks like it's an option they've enabled in their recent 64 bit kernels (Quantal+)
You need a completely different userland, compiled for x32 instead of amd64. Just running the default amd64 installation with a x32 kernel doesn't make it x32. EDIT: It would also work for just running x32 apps on an amd64 system, but then you only get the x32 benefits for those applications.
People are finally getting around to experimenting now there is gcc support. You need an x32 libc too, but you can use a chroot environment.
I realized after writing my post that you don't really need an entire system, just the x32 libraries (and not even those if you statically link your x32 application).
You don't need a whole x32 userland in order to be exploited. Just a single x32 binary executable that happens to not depend on on any external libraries.
You don't even need that. This can be done within a 64bit process using the x32 syscall number for recvmmsg.
-Brad
Ouch.
I did't mean for an exploit, I just meant that you don't have any use for a x32 kernel on your system if you don't have x32 apps and libraries (unless you build a statically-linked x32 app).