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

[Jann Horn]

On Fri, May 5, 2017 at 5:49 PM, Jann Horn <jannh@xxxxxxxxxx> wrote:
> 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?

Ah, I think I understand. The kernel stacks are mapped, but
cpu_current_top_of_stack isn't, so you can't find the stack until after the CR3
switch in the syscall handler?