linux-kernel - Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

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Date:   Fri, 5 Jan 2018 14:16:38 +0800
From:   Yisheng Xie <xieyisheng1@...wei.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        <linux-kernel@...r.kernel.org>
CC:     <linux-mm@...ck.org>, <richard.fellner@...dent.tugraz.at>,
        <moritz.lipp@...k.tugraz.at>, <daniel.gruss@...k.tugraz.at>,
        <michael.schwarz@...k.tugraz.at>, <luto@...nel.org>,
        <torvalds@...ux-foundation.org>, <keescook@...gle.com>,
        <hughd@...gle.com>, <x86@...nel.org>
Subject: Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page
 tables (core patch)

Hi Dave,

On 2018/1/5 13:18, Dave Hansen wrote:
> On 01/04/2018 08:16 PM, Yisheng Xie wrote:
>>> === Page Table Poisoning ===
>>>
>>> KAISER has two copies of the page tables: one for the kernel and
>>> one for when running in userspace.  
>>
>> So, we have 2 page table, thinking about this case:
>> If _ONE_ process includes _TWO_ threads, one run in user space, the other
>> run in kernel, they can run in one core with Hyper-Threading, right?
> 
> Yes.
> 
>> So both userspace and kernel space is valid, right? And for one core
>> with Hyper-Threading, they may share TLB, so the timing problem
>> described in the paper may still exist?
> 
> No.  The TLB is managed per logical CPU (hyperthread), as is the CR3
> register that points to the page tables.  Two threads running the same
> process might use the same CR3 _value_, but that does not mean they
> share TLB entries.

Get it, and thanks for your explain.

BTW, we have just reported a bug caused by kaiser[1], which looks like
caused by SMEP. Could you please help to have a look?

[1] https://lkml.org/lkml/2018/1/5/3

Thanks
Yisheng

> 
> One thread *can* be in the kernel with the kernel page tables while the
> other is in userspace with the user page tables active.  They will even
> use a different PCID/ASID for the same page tables normally.
> 
>> Can this case still be protected by KAISER?
> 
> Yes.
> 
> .
>