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SubjectRe: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs
----- On Jul 3, 2018, at 1:34 PM, Peter Zijlstra 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 <> 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 ?



Mathieu Desnoyers
EfficiOS Inc.

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