Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20201116144757.1920077-1-alexandre.chartre@oracle.com> <20201116144757.1920077-22-alexandre.chartre@oracle.com>
In-Reply-To: <20201116144757.1920077-22-alexandre.chartre@oracle.com>
From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 16 Nov 2020 08:56:13 -0800
Message-ID: <CALCETrU5qnsxgLb6W5UPk8RRbbkFTTbjLxHefkBXCyk-2=uDcQ@mail.gmail.com>
Subject: Re: [RFC][PATCH v2 21/21] x86/pti: Use a different stack canary with
 the user and kernel page-table
To:     Alexandre Chartre <alexandre.chartre@oracle.com>
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>,
        Andrew Lutomirski <luto@kernel.org>,
        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
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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