linux-kernel - Re: [RFC PATCH for 4.15 09/14] Provide cpu

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Message-ID: <112864193.41278.1507990930227.JavaMail.zimbra@efficios.com>
Date:   Sat, 14 Oct 2017 14:22:10 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        Josh Triplett <josh@...htriplett.org>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH for 4.15 09/14] Provide cpu_opv system call

----- On Oct 13, 2017, at 10:50 AM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:

> ----- On Oct 13, 2017, at 9:57 AM, One Thousand Gnomes
> gnomes@...rguk.ukuu.org.uk wrote:
> 
>>> A maximum limit of 16 operations per cpu_opv syscall invocation is
>>> enforced, so user-space cannot generate a too long preempt-off critical
>>> section.
>> 
>> Except that all the operations could be going to mmapped I/O space and if
>> I pick the right targets could take quite a long time to complete.
> 
> We could check whether a struct page belongs to mmapped I/O space, and return
> EINVAL in that case.
> 
>> It's
>> still only 16 operations - But 160ms is a lot worse than 10ms. In fact
>> with compare_iter I could make it much much worse still as I get 2 x
>> TMP_BUFLEN x 16 x worst case latency in my attack. That's enough to screw
>> up plenty of things.
> 
> Would a check that ensures the page is not mmapped I/O space be sufficient
> to take care of this ? If happen to know which API I need to look for, it
> would be welcome.

I think is_zone_device_page() is what I was looking for.

Let me know if I missed something,

Thanks,

Mathieu

> Thanks,
> 
> Mathieu
> 
> 
>> 
>> Alan
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com