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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Message-ID: <3cf22df6-86fd-91b3-6dde-ce28ca48a6f6@oracle.com>
Date:   Mon, 16 Nov 2020 19:34:50 +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 21/21] x86/pti: Use a different stack canary with
 the user and kernel page-table


On 11/16/20 5:56 PM, Andy Lutomirski wrote:
> 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 you are right because we have the mapping to the stack canary in
the user page-table. From userspace, we will only read the user stack canary,
but using Meltdown we can speculatively read the kernel stack canary which
will be stored at the same place.

> I think that doing this safely requires mapping a different page with
> the stack canary in the two pagetables.

Right.

alex.