linux-kernel - Re: [tip:sched/core] sched/core: Fix a potential double-fetch bug in sched_copy

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Date:   Sun, 10 Mar 2019 11:09:53 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     mingo@...nel.org, kjlu@....edu, hpa@...or.com,
        stable@...r.kernel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched/core: Fix a potential double-fetch bug
 in sched_copy_attr()

On Mon, 28 Jan 2019, Thomas Gleixner wrote:
> On Mon, 28 Jan 2019, Peter Zijlstra wrote:
> > 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.
> 
> Yes please.
> 
> The point is that by default the data passed to a function (and it does not
> matter whether it's a syscall) by pointer is immutable. There is exactly
> ONE syscall which is specifically designed to deal with mutable data and
> that's a constant source of headache ....
> 
> Kangjie is right that all double fetch operations like the one in
> sched_copy_attr() are prone to concurrent modification problems. But then
> we really should say NO instead of silently trying to fix things up. I
> personally would even kill the process immediately, no matter whether the
> corruption is caused by malicious intent or by sheer stupidity.

Just noticed that this fell through the cracks. Is anyone working on a fix
for this?

Thanks,

	tglx