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

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Message-ID: <20180117200532.GX13338@ZenIV.linux.org.uk>
Date:   Wed, 17 Jan 2018 20:05:32 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Alan Cox <alan@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        "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 Wed, Jan 17, 2018 at 11:54:12AM -0800, Dan Williams wrote:
> On Wed, Jan 17, 2018 at 10:52 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > On Wed, Jan 17, 2018 at 02:17:26PM +0000, Alan Cox wrote:
> [..]
> >         Incidentally, what about copy_to_iter() and friends?  They
> > check iov_iter flavour and go either into the "copy to kernel buffer"
> > or "copy to userland" paths.  Do we need to deal with mispredictions
> > there?  We are calling a bunch of those on read()...
> >
> 
> Those should be protected by the conversion of __uaccess_begin to
> __uaccess_begin_nospec that includes the lfence.

Huh?  What the hell does it do to speculative execution of "memcpy those
suckers" branch?