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: <cover.1620186182.git.jpoimboe@redhat.com> <5ba93cdbf35ab40264a9265fc24575a9b2f813b3.1620186182.git.jpoimboe@redhat.com>
 <2f75c496ac774444b75ff808854b8e5f@AcuMS.aculab.com>
In-Reply-To: <2f75c496ac774444b75ff808854b8e5f@AcuMS.aculab.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 5 May 2021 11:32:17 -0700
Message-ID: <CAHk-=wh-a6B11tH3upLG+-P5_yFPs3PB8tiXO5JKaQjUvhOMYg@mail.gmail.com>
Subject: Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation
To:     David Laight <David.Laight@aculab.com>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Will Deacon <will@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Mark Rutland <mark.rutland@arm.com>,
        Borislav Petkov <bp@alien8.de>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, May 5, 2021 at 1:48 AM David Laight <David.Laight@aculab.com> wrote:
>
> This would error requests for address 0 earlier - but I don't
> believe they are ever valid in Linux.
> (Some historic x86 a.out formats did load to address 0.)

Not only loading at address 0 - there are various real reason s why
address 0 might actually be needed.

Anybody who still runs a 32-bit kernel and wants to use vm86 mode, for
example, requires address 0 because that's simply how the hardware
works.

So no. "mask to zero and make zero invalid" is not a proper model.

            Linus