Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs

From: Peter Zijlstra
Date: Tue Jul 03 2018 - 13:35:54 EST

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@xxxxxxxxxxxxxx> 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.