linux-kernel - Re: [RFC][PATCH v2 11/21] x86/pti: Extend PTI user mappings

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Message-ID: <820278dc-5f8e-6224-71b4-7c61819f68d1@oracle.com>
Date:   Mon, 16 Nov 2020 21:21:03 +0100
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        jan.setjeeilers@...cle.com, Junaid Shahid <junaids@...gle.com>,
        oweisse@...gle.com, Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Alexander Graf <graf@...zon.de>, mgross@...ux.intel.com,
        kuzuno@...il.com
Subject: Re: [RFC][PATCH v2 11/21] x86/pti: Extend PTI user mappings


On 11/16/20 8:48 PM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 6:49 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, per cpu offsets (__per_cpu_offset, which is used some in
>> entry code), the stack canary, and the PTI stack (which is defined per
>> task).
> 
> Does anything unmap the PTI stack?  Mapping is easy, and unmapping
> could be a pretty big mess.
> 

No, there's no unmap. The mapping exists as long as the task page-table
does (i.e. as long as the task mm exits). I assume that the task stack
and mm are freed at the same time but that's not something I have checked.

alex.