linux-kernel - Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq

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Message-Id: <6C9EE370-1BE4-4556-8B79-39BDBBAED7F8@amacapital.net>
Date:   Tue, 3 Jul 2018 10:06:40 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>, michal.simek@...inx.com,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs


On Jul 3, 2018, at 9:40 AM, Andi Kleen <andi@...stfloor.org> wrote:

>> 
>> So I think you're good... But yes, you raise an interresting point.
> 
> So it sounds like architectures that don't have an instruction atomic u64
> *_user need to disable interrupts during the access, and somehow handle that
> case when a page fault happens?

I think all this discussion of “atomic” is a huge distraction. The properties we need are:

- User code can change rseq_cs from one valid user pointer to another with a single instruction (or equivalent) such that we can’t end up in the kernel with the write only partially done as seen in that thread.

- The kernel needs to be able to read the value consistently with the above requirement.

I don’t think it’s possible to have a valid implementation of get_user() on any architecture that’s so weak that this doesn’t work.

If user code writes rseq_cs from the wrong thread, I think the user code is buggy and we simply don’t care what happens.  The kernel should be allowed to use an arbitrarily weak read with respect to other threads.