linux-kernel - Re: [RFC][PATCH 13/24] x86/pti: Extend PTI user mappings

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Date:   Mon, 9 Nov 2020 18:52:55 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 13/24] x86/pti: Extend PTI user mappings

On 11/9/20 6:28 PM, Andy Lutomirski wrote:
> On Mon, Nov 9, 2020 at 3:22 AM Alexandre Chartre
> <alexandre.chartre@...cle.com> wrote:
>>
>> Extend PTI user mappings so that more kernel entry code can be executed
>> with the user page-table. To do so, we need to map syscall and interrupt
>> entry code,
> 
> Probably fine.
> 
>> per cpu offsets (__per_cpu_offset, which is used some in
>> entry code),
> 
> This likely already leaks due to vulnerable CPUs leaking address space
> layout info.

I forgot to update the comment, I am not mapping __per_cpu_offset anymore.

However, if we do map __per_cpu_offset then we don't need to enforce the
ordering in paranoid_entry to switch CR3 before GS.

> 
>> the stack canary,
> 
> That's going to be a very tough sell.
> 

I can get rid of this, but this will require to disable stack-protector for
any function that we can call while using the user page-table, like already
done in patch 21 (x86/entry: Disable stack-protector for IST entry C handlers).

alex.