linux-kernel - Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation

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Date:   Wed, 5 May 2021 11:32:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Mark Rutland <mark.rutland@....com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation

On Wed, May 5, 2021 at 1:48 AM David Laight <David.Laight@...lab.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