netdev - Re: [PATCH 14/18] ipv4: prevent bounds-check bypass via speculative execution

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Date:   Sat, 6 Jan 2018 10:10:38 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Netdev <netdev@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 14/18] ipv4: prevent bounds-check bypass via speculative execution

On Sat, Jan 6, 2018 at 8:29 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Sat, Jan 6, 2018 at 7:14 AM, Greg KH <gregkh@...uxfoundation.org> wrote:
>> On Sat, Jan 06, 2018 at 12:23:47PM +0000, Alan Cox wrote:
>>> On Sat, 6 Jan 2018 10:01:54 +0100
>>> Greg KH <gregkh@...uxfoundation.org> wrote:
>>>
>>> > On Fri, Jan 05, 2018 at 05:11:10PM -0800, Dan Williams wrote:
>>> > > Static analysis reports that 'offset' may be a user controlled value
>>> >
>>> > Can I see the rule that determined that?  It does not feel like that is
>>> > correct, given the 3+ levels deep that this function gets this value
>>> > from...
>>>
>>> On a current x86 you can execute something upwards of 150 instructions in
>>> a speculation window.
>>
>> Yeah, I agree, it's deep :(
>>
>> But for this patch, I thought the prior review determined that it was
>> not a problem.  Was that somehow proven incorrect?
>
> I kept it in the series to get a re-review with the wider netdev
> because I missed the discussion leading up to that 'drop the patch'
> decision. Sorry, I should have noted that in the changelog or cover
> letter.

Is there a microbenchmark that can stress this path. If the cost is
negligible why play games with it being "probably ok"? Unless someone
can say 100% 'offset' is always bounded to something safe by the time
we get here just use nospec_array_ptr().