linux-kernel - Re: [RFC][PATCH v2 21/21] x86/pti: Use a different stack canary with the user and kernel page-table

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Date:   Mon, 16 Nov 2020 08:56:13 -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 v2 21/21] x86/pti: Use a different stack canary with
 the user and kernel page-table

On Mon, Nov 16, 2020 at 6:48 AM Alexandre Chartre
<alexandre.chartre@...cle.com> wrote:
>
> Using stack protector requires the stack canary to be mapped into
> the current page-table. Now that the page-table switch between the
> user and kernel page-table is deferred to C code, stack protector can
> be used while the user page-table is active and so the stack canary
> is mapped into the user page-table.
>
> To prevent leaking the stack canary used with the kernel page-table,
> use a different canary with the user and kernel page-table. The stack
> canary is changed when switching the page-table.

Unless I've missed something, this doesn't have the security
properties we want.  One CPU can be executing with kernel CR3, and
another CPU can read the stack canary using Meltdown.  I think that
doing this safely requires mapping a different page with the stack
canary in the two pagetables.