Re: [tip:sched/core] sched/core: Fix a potential double-fetch bug in sched_copy_attr()

[Peter Zijlstra]

On Sun, Jan 27, 2019 at 12:04:44PM +0100, Thomas Gleixner wrote:
> On Mon, 21 Jan 2019, tip-bot for Kangjie Lu wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index a674c7db2f29..d4d3514c4fe9 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4499,6 +4499,9 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
> >  	if (ret)
> >  		return -EFAULT;
> >  
> > +	/* In case attr->size was changed by user-space: */
> > +	attr->size = size;
> > +
> 
> Just when pondering to send that to Linus, I tried to write up a concise
> summary for this which made me look at the patch.
> 
> If the size changed, then its clear that user space fiddled with the date
> between the size fetch and the full copy from user. So why restoring the
> size instead of doing the obvious:
> 
>    	 if (attr->size != size)
> 	 	return -ECRAP;
> 
> Hmm?

Sure; but if we do that we should also change perf_copy_attr() which has
the exact same thing.