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Date:   Tue, 3 Jul 2018 19:34:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andi Kleen <andi@firstfloor.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Paul 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 - ARM Linux <linux@arm.linux.org.uk>,
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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 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.