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Subject: Re: [RFC][PATCH v2 21/21] x86/pti: Use a different stack canary with
 the user and kernel page-table
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Joerg Roedel <jroedel@suse.de>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        jan.setjeeilers@oracle.com, Junaid Shahid <junaids@google.com>,
        oweisse@google.com, Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Alexander Graf <graf@amazon.de>, mgross@linux.intel.com,
        kuzuno@gmail.com
References: <20201116144757.1920077-1-alexandre.chartre@oracle.com>
 <20201116144757.1920077-22-alexandre.chartre@oracle.com>
 <CALCETrU5qnsxgLb6W5UPk8RRbbkFTTbjLxHefkBXCyk-2=uDcQ@mail.gmail.com>
From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Message-ID: <3cf22df6-86fd-91b3-6dde-ce28ca48a6f6@oracle.com>
Date:   Mon, 16 Nov 2020 19:34:50 +0100
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On 11/16/20 5:56 PM, Andy Lutomirski wrote:
> On Mon, Nov 16, 2020 at 6:48 AM Alexandre Chartre
> <alexandre.chartre@oracle.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.