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

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Date:   Tue, 3 Jul 2018 10:43:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        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>, Andi Kleen <andi@...stfloor.org>,
        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 Tue, Jul 03, 2018 at 10:29:55AM +0200, Heiko Carstens wrote:
> On Tue, Jul 03, 2018 at 10:14:49AM +0200, Peter Zijlstra wrote:
> > On Mon, Jul 02, 2018 at 10:30:09PM -0400, Mathieu Desnoyers wrote:
> > > > Use "get_user()". It works for 64-bit objects too, and it will be
> > > > atomic in the 32-bit sub-parts on a 32-bit architecture.
> > > 
> > > Is it really ? Last time we had this discussion, not all architectures
> > > guaranteed that reading a 64-bit integer would happen in two atomic
> > > 32-bit sub-parts. This was the main motivation for the LINUX_FIELD_u32_u64()
> > > macro as it stands today (rather than using a union).
> > 
> > Just state, as a requirement for supporting rseq, that the arch
> > {get,put}_user(u64) on 32bit targets must be exactly 2 u32 loads/stores.
> > 
> > We're piece-wise enabling rseq across architectures anyway, and when the
> > relevant maintains do this, they can have a look at their
> > {get,put}_user() implementations and fix them.
> > 
> > If you rely on get_user(u64) working, that means microblaze is already
> > broken, but I suppose it already was, since their rseq enablement patch
> > is extremely dodgy. Michal?
> 
> s390 uses the mvcos instruction to implement get_user(). That instruction
> is not defined to be atomic, but may copy bytes piecemeal.. I had the
> impression that the rseq fields are supposed to be updated within the
> context of a single thread (user + kernel space).
> 
> However if another user space thread is allowed to do this as well, then
> the get_user() approach won't fly on s390.
> 
> That leaves the question: does it even make sense for a thread to update
> the rseq structure of a different thread?

The problem is interrupts; we need interrupts on the CPU doing the store
to observe either the old or the new value, not a mix.

If mvcos does not guarantee that, we're having problems. Is there a
reason get_user() cannot use a 'regular' load?