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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 2:28 PM, Peter Zijlstra peterz@infradead.org wrote:

> On Tue, Jul 03, 2018 at 02:15:34PM -0400, Mathieu Desnoyers wrote:
>> ----- On Jul 3, 2018, at 2:11 PM, Peter Zijlstra peterz@infradead.org wrote:
>>
>> > On Tue, Jul 03, 2018 at 01:58:37PM -0400, Mathieu Desnoyers wrote:
>> >> I can modify the ABI to put the cpu_id_start and cpu_id fields inside
>> >> a union, and update it with a single store.
>> >>
>> >> Thoughts ?
>> >
>> > Let's keep them for now, we can always frob this later, they are aligned
>> > and proper, no need to expose that union to userspace.
>>
>> Isn't it weird to change the API of an exposed public uapi header ?
>
> Sure, just keep it as is. We don't need an exposed union to do a single
> store there.
>
> Something like the ugly below preserves API but still does a single
> store.
>
> But sure, if you want to expose that union for some reason, then now is
> the time.

User-space won't ever want to read cpu_id_start and cpu_id from a single
u64 load, it serves no purpose to do so. So I'm OK with keeping those as
is and defining a local union for the __put_user() update.

Thanks!

Mathieu

>
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 22b6acf1ad63..e956c48b5f83 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -85,10 +85,17 @@ static int rseq_update_cpu_id(struct task_struct *t)
> {
> u32 cpu_id = raw_smp_processor_id();
>
> - if (__put_user(cpu_id, &t->rseq->cpu_id_start))
> - return -EFAULT;
> - if (__put_user(cpu_id, &t->rseq->cpu_id))
> + union {
> + struct {
> + u32 cpu_id_start;
> + u32 cpu_id;
> + };
> + u64 val;
> + } x = { { .cpu_id_start = cpu_id, .cpu_id = cpu_id, } };
> +
> + if (__put_user(x.val, (u64 *)&t->rseq->cpu_id_start))
> return -EFAULT;
> +
> trace_rseq_update(t);
> return 0;
> }

--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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