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

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Date:   Wed, 17 Jan 2018 14:17:26 +0000
From:   Alan Cox <alan@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE
 in get_user paths

On Tue, 2018-01-16 at 14:41 -0800, Linus Torvalds wrote:
> 
> 
> On Jan 16, 2018 14:23, "Dan Williams" <dan.j.williams@...el.com>
> wrote:
> > That said, for get_user specifically, can we do something even
> > cheaper. Dave H. reminds me that any valid user pointer that gets
> > past
> > the address limit check will have the high bit clear. So instead of
> > calculating a mask, just unconditionally clear the high bit. It
> > seems
> > worse case userspace can speculatively leak something that's
> > already
> > in its address space.
> 
> That's not at all true.
> 
> The address may be a kernel address. That's the whole point of
> 'set_fs()'.

Can we kill off the remaining users of set_fs() ?

Alan