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Date:   Tue, 3 Jul 2018 19:48:33 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <andi@firstfloor.org>,
        heiko carstens <heiko.carstens@de.ibm.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Chris Lameter <cl@linux.com>,
        Ben Maurer <bmaurer@fb.com>, rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
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        Michal Simek <michal.simek@xilinx.com>,
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Subject: Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields,
 validate user inputs
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On Tue, Jul 03, 2018 at 01:38:59PM -0400, Mathieu Desnoyers wrote:
> ----- On Jul 3, 2018, at 1:34 PM, Peter Zijlstra peterz@infradead.org wrote:
> 
> > On Tue, Jul 03, 2018 at 10:10:37AM -0700, Linus Torvalds wrote:
> >> On Tue, Jul 3, 2018 at 9:40 AM Andi Kleen <andi@firstfloor.org> wrote:
> >> >
> >> > 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?
> >> 
> >> No. It's actually the store by *user* space that is the critical one.
> >> Not the whole 64-bit value, just the low pointer part.
> >> 
> >> The kernel could do it as a byte-by-byte load, really. It's
> >> per-thread, and once the kernel is running, it's not going to change.
> >> The kernel never changes the value, it just loads it from user space.
> > 
> > The kernel doesn't change _this_ value, but the kernel does change other
> > values, like for instance rseq->cpu_id. But even there, it could use
> > byte stores and it is again the userspace load of that field that is
> > critical again and needs to be a single op.
> 
> I can simply document that loads/stores from/to all struct rseq fields
> should be thread-local then ?

I'm not sure that covers things sufficiently. You really want the
userspace load/stores to be single instructions.

Also, I think it was rseq_update_cpu_id() where we wanted to use a
single u64 store if possible but you worried about the stores.