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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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