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Message-ID: <CALCETrU5qnsxgLb6W5UPk8RRbbkFTTbjLxHefkBXCyk-2=uDcQ@mail.gmail.com>
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.
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