Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map kernel in user mode

From: Jann Horn
Date: Fri May 05 2017 - 11:49:41 EST

On Thu, May 4, 2017 at 12:02 PM, Daniel Gruss
<daniel.gruss@xxxxxxxxxxxxxx> wrote:
> After several recent works [1,2,3] KASLR on x86_64 was basically considered
> dead by many researchers. We have been working on an efficient but effective
> fix for this problem and found that not mapping the kernel space when
> running in user mode is the solution to this problem [4] (the corresponding
> paper [5] will be presented at ESSoS17).
> With this RFC patch we allow anybody to configure their kernel with the flag
> CONFIG_KAISER to add our defense mechanism.
> If there are any questions we would love to answer them.
> We also appreciate any comments!

Why do you need this SWITCH_KERNEL_CR3_NO_STACK logic? It would
make sense if the kernel stacks weren't mapped, but if they weren't mapped,
I don't see how the entry_INT80_compat entry point could work at all - the
software interrupt itself already pushes values on the kernel stack. You could
maybe work around that using some sort of trampoline stack, but I don't see
anything like that. Am I missing something?