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 [off-list ref] wrote:

quoted

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.