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

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Date:   Tue, 9 Jan 2018 15:41:49 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
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>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

On Fri, Jan 05, 2018 at 06:52:07PM -0800, Linus Torvalds 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.
> 
> The description is pure garbage afaik.
> 
> The fact is, we have to stop speculating when access_ok() does *not*
> fail - because that's when we'll actually do the access. And it's that
> access that needs to be non-speculative.
> 
> That actually seems to be what the code does (it stops speculation
> when __range_not_ok() returns false, but access_ok() is
> !__range_not_ok()). But the explanation is crap, and dangerous.

The description also seems to be missing the "why", as it's not
self-evident (to me, at least).

Isn't this (access_ok/uaccess_begin/ASM_STAC) too early for the lfence?

i.e., wouldn't the pattern be:

	get_user(uval, uptr);
	if (uval < array_size) {
		lfence();
		foo = a2[a1[uval] * 256];
	}

Shouldn't the lfence come much later, *after* reading the variable and
comparing it and branching accordingly?

-- 
Josh