linux-kernel - Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER

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Date:   Fri, 5 May 2017 17:53:30 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Daniel Gruss <daniel.gruss@...k.tugraz.at>
Cc:     kernel list <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        "clementine.maurice@...k.tugraz.at" 
        <clementine.maurice@...k.tugraz.at>,
        "moritz.lipp@...k.tugraz.at" <moritz.lipp@...k.tugraz.at>,
        Michael Schwarz <michael.schwarz@...k.tugraz.at>,
        Richard Fellner <richard.fellner@...dent.tugraz.at>,
        kirill.shutemov@...ux.intel.com, Ingo Molnar <mingo@...nel.org>,
        "anders.fogh@...ta-adan.de" <anders.fogh@...ta-adan.de>
Subject: Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map
 kernel in user mode

On Fri, May 5, 2017 at 5:49 PM, Jann Horn <jannh@...gle.com> wrote:
> On Thu, May 4, 2017 at 12:02 PM, Daniel Gruss
> <daniel.gruss@...k.tugraz.at> 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?