Le jeudi 22 avril 2010 à 08:51 -0700, Paul E. McKenney a écrit : > On Thu, Apr 22, 2010 at 04:53:49PM +0200, Eric Dumazet wrote: > > Le jeudi 22 avril 2010 à 16:36 +0200, Eric Dumazet a écrit : > > > > > If one hash slot is under attack, then there is a bug somewhere. > > > > > > If we cannot avoid this, we can fallback to a secure mode at the second > > > retry, and take the spinlock. > > > > > > Tis way, most of lookups stay lockless (one pass), and some might take > > > the slot lock to avoid the possibility of a loop. > > > > > > I suspect a bug elsewhere, quite frankly ! > > > > > > We have a chain that have an end pointer that doesnt match the expected > > > one. > > > > > > > On normal situation, we always finish the lookup : > > > > 1) If we found the thing we were looking at. > > > > 2) We get the list end (item not found), we then check if it is the > > expected end. > > > > It is _not_ the expected end only if some writer deleted/inserted an > > element in _this_ chain during our lookup. > > So this situation uses SLAB_DESTROY_BY_RCU to quickly recycle deleted > elements? (Not obvious from the code, but my ignorance of the networking > code is such that many things in that part of the kernel are not obvious > to me, I am afraid.) > Yes, this uses SLAB_DESTROY_BY_RCU, like tcp/udp lookups. > Otherwise, of course you would simply allow deleted elements to continue > pointing where they did previously, so that concurrent readers would not > miss anything. > > Of course, the same potential might arise on insertion, but it is usually > OK to miss an element that was inserted after you started searching. > > > Because our lookup is lockless, we then have to redo it because we might > > miss the object we are looking for. > > Ah... Is there also a resize operation? Herbert did do a resizable > hash table recently, but I was under the impression that (1) it was in > some other part of the networking stack and (2) it avoided the need to > restart readers. > > > If we can do the 'retry' a 10 times, it means the attacker was really > > clever enough to inject new packets (new conntracks) at the right > > moment, in the right hash chain, and this sounds so higly incredible > > that I cannot believe it at all :) > > Or maybe the DoS attack is injecting so many new conntracks that a large > fraction of the hash chains are being modified at any given time? > > Thanx, Paul maybe hash table has one slot :) -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
