On Mon, Jul 15, 2019 at 03:33:11PM +0200, Dmitry Vyukov wrote:

On Mon, Jul 15, 2019 at 3:29 PM Peter Zijlstra [off-list ref] wrote:

quoted

On Sun, Jul 14, 2019 at 11:49:15AM -0700, Paul E. McKenney wrote:

quoted

On Sun, Jul 14, 2019 at 05:48:00PM +0300, Dmitry Vyukov wrote:

quoted

But short term I don't see any other solution than stop testing
sched_setattr because it does not check arguments enough to prevent
system misbehavior. Which is a pity because syzkaller has found some
bad misconfigurations that were oversight on checking side.
Any other suggestions?

Keep the times down to a few seconds?  Of course, that might also
fail to find interesting bugs.

Right, if syzcaller can put a limit on the period/deadline parameters
(and make sure to not write "-1" to
/proc/sys/kernel/sched_rt_runtime_us) then per the in-kernel
access-control should not allow these things to happen.

Since we are racing with emails, could you suggest a 100% safe
parameters? Because I only hear people saying "safe", "sane",
"well-behaving" :)
If we move the check to user-space, it does not mean that we can get
away without actually defining what that means.

Right, well, that's part of the problem. I think Paul just did the
reverse math and figured that 95% of X must not be larger than my
watchdog timeout and landed on 14 seconds.

I'm thinking 4 seconds (or rather 4.294967296) would be a very nice
number.

Now thinking of this, if we come up with some simple criteria, could
we have something like a sysctl that would allow only really "safe"
parameters?

I suppose we could do that, something like:
sysctl_deadline_period_{min,max}. I'll have to dig back a bit on where
we last talked about that and what the problems where.

For one, setting the min is a lot harder, but I suppose we can start at
TICK_NSEC or something.