linux-kernel - Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get

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Message-ID: <CA+55aFxdxtScVaSHU9_8v+r719r03ZHJ7KLB0CTHm8NahRgt=A@mail.gmail.com>
Date:   Wed, 17 Jan 2018 11:16:50 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
 get_user paths

On Tue, Jan 16, 2018 at 8:30 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>
> I think the access_ok() conversion to return a speculation sanitized
> pointer or NULL is the way to go unless I'm missing something simpler.

No, that's way too big of a conversion.

Just make get_user() and friends (that currently use ASM_STAC) use the
address masking.

The people who use uaccess_begin() can use the lfence there.

Basically, the rule is trivial: find all 'stac' users, and use address
masking if those users already integrate the limit check, and lfence
they don't.

                Linus