linux-kernel - Re: [PATCH 06/18] x86, barrier: stop speculation for failed access

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Message-ID: <20180106123242.77f4d860@alans-desktop>
Date:   Sat, 6 Jan 2018 12:32:42 +0000
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Network Development <netdev@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed
 access_ok

On Fri, 5 Jan 2018 18:52:07 -0800
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Fri, Jan 5, 2018 at 5:10 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> > From: Andi Kleen <ak@...ux.intel.com>
> >
> > When access_ok fails we should always stop speculating.
> > Add the required barriers to the x86 access_ok macro.  
> 
> Honestly, this seems completely bogus.

Also for x86-64 if we are trusting that an AND with a constant won't get
speculated into something else surely we can just and the address with ~(1
<< 63) before copying from/to user space ? The user will then just
speculatively steal their own memory.

Alan