On Mon, 16 Mar 2015 08:49:40 -0700 "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> On Mon, Mar 16, 2015 at 10:24:30AM -0400, Steven Rostedt wrote: > > On Mon, 16 Mar 2015 15:19:39 +0100 > > Peter Zijlstra <pet...@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? > That I wasn't replying about the patch but only to Peter's comment, which made it appear that sys_membar() would spew IPIs over the entire system. If this is not a case, then there's no need for rate limiting. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
