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

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Date:   Tue, 9 Jan 2018 13:47:09 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     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>,
        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 Tue, Jan 9, 2018 at 1:41 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 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?

The goal of putting the lfence in uaccess_begin() is to prevent
speculation past access_ok(). You are correct that the cpu could later
mis-speculate on uval, that's where taint analysis tooling needs to
come into play to track uval to where it is used. That's where the
nospec_array_ptr() patches come into play.