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

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Date:   Tue, 3 Jul 2018 19:34:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Paul 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 - ARM Linux <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
        Christoph Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        Steven 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 <michal.simek@...inx.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>, 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 10:10:37AM -0700, Linus Torvalds wrote:
> On Tue, Jul 3, 2018 at 9:40 AM Andi Kleen <andi@...stfloor.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.