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

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Date:   Tue, 10 Nov 2020 15:39:43 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
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>,
        Andrew Lutomirski <luto@...nel.org>,
        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 13/24] x86/pti: Extend PTI user mappings

On Mon, Nov 9, 2020 at 11:54 AM Alexandre Chartre
<alexandre.chartre@...cle.com> wrote:
>
>
> [Copying the reply to Andy in the thread with the right email addresses]
>
> 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.

I'm okay with mapping __per_cpu_offset.

>
> >
> >> 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).
>

You could probably get away with using a different stack protector
canary before and after the CR3 switch as long as you are careful to
have the canary restored when you return from whatever function is
involved.