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: [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  (the corresponding >> paper  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?
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