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

[Ingo Molnar <mingo@kernel.org>]

* Thomas Gleixner <tglx@linutronix.de> wrote:

> On Mon, 21 Jan 2019, tip-bot for Kangjie Lu wrote:
> > Commit-ID:  120e4e76857ddbc9268e1aa3f9de61a498e84618
> > Gitweb:     https://git.kernel.org/tip/120e4e76857ddbc9268e1aa3f9de61a498e84618
> > Author:     Kangjie Lu <kjlu@umn.edu>
> > AuthorDate: Wed, 9 Jan 2019 01:45:24 -0600
> > Committer:  Ingo Molnar <mingo@kernel.org>
> > CommitDate: Mon, 21 Jan 2019 11:26:17 +0100
> > 
> > sched/core: Fix a potential double-fetch bug in sched_copy_attr()
> > 
> > "uattr->size" is copied in from user space and checked. However, it is
> > copied in again after the security check. A malicious user may race to
> > change it. The fix sets uattr->size to be the checked size.
> > 
> > Signed-off-by: Kangjie Lu <kjlu@umn.edu>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: pakki001@umn.edu
> > Cc: <stable@vger.kernel.org>
> > Link: https://lkml.kernel.org/r/20190109074524.10176-1-kjlu@umn.edu
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> > ---
> >  kernel/sched/core.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > 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?

Yeah, indeed - and that's a much more reliable interface behavior in any 
case. It's probably also faster.

Thanks,

	Ingo