Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier (x86) (v12)

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Mar 16, 2015 at 10:24:30AM -0400, Steven Rostedt wrote:
> On Mon, 16 Mar 2015 15:19:39 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > 
> > I suppose this is an unprivileged syscall; so what do we do about:
> > 
> > 	for (;;)
> > 		sys_membar(EXPEDITED);
> > 
> > Which would spray the entire system with IPIs at break neck speed.
> 
> Perhaps it should be rate limited. Have parameters (controlled via
> sysctl) that will only allow so many of these per ms. If it exceeds it,
> then the call will end up being a schedule_timeout() till it is allowed
> to continue. Thus, the above will spit out a few hundred IPIs, then
> sleep for a millisecond, and then spit out another hundred IPIs and
> sleep again.
> 
> That would prevent any DoS attacks.

But this would only qualify as a DoS if MEMBARRIER_EXPEDITED_FLAG and
!MEMBARRIER_PRIVATE_FLAG.  Otherwise, the user's process is only DoSing
itself, which is that user's problem, not anyone else's.  And it looks
like the current patch refuses to implement this DoS case, unless I am
really confused about the code in membarrier_expedited().  And in fact
membarrier_validate_flags() checks for this DoS case and returns -EINVAL.

So I do not believe that this syscall permits that type of DoS.

What am I missing here?

							Thanx, Paul