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

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Date:	Mon, 16 Mar 2015 08:49:40 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Nicholas Miell <nmiell@...cast.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier
 (x86) (v12)

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@...radead.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

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