On Wed, Aug 07, 2013 at 04:37:27PM +0200, Michal Hocko wrote: > On Wed 07-08-13 09:58:18, Tejun Heo wrote: > > Hello, > > > > On Wed, Aug 07, 2013 at 03:46:54PM +0200, Michal Hocko wrote: > > > OK, I have obviously misunderstood your concern mentioned in the other > > > email. Could you be more specific what is the DoS scenario which was > > > your concern, then? > > > > So, let's say the file is write-accessible to !priv user which is > > under reasonable resource limits. Normally this shouldn't affect priv > > system tools which are monitoring the same event as it shouldn't be > > able to deplete resources as long as the resource control mechanisms > > are configured and functioning properly; however, the memory usage > > event puts all event listeners into a single contiguous table which a > > !priv user can easily expand to a size where the table can no longer > > be enlarged and if a priv system tool or another user tries to > > register event afterwards, it'll fail. IOW, it creates a shared > > resource which isn't properly provisioned and can be trivially filled > > up making it an easy DoS target. > > OK, got your point. You are right and I haven't considered the size of > the table and the size restrictions of kmalloc. Thanks for pointing this > out! > --- > From cde8a3333296eddd288780e78803610127401b6a Mon Sep 17 00:00:00 2001 > From: Michal Hocko <mhocko@xxxxxxx> > Date: Wed, 7 Aug 2013 11:11:22 +0200 > Subject: [PATCH] memcg: limit the number of thresholds per-memcg > > There is no limit for the maximum number of threshold events registered > per memcg. It is even worse that all the events are stored in a > per-memcg table which is enlarged when a new event is registered. This > can lead to the following issue mentioned by Tejun: > " > So, let's say the file is write-accessible to !priv user which is > under reasonable resource limits. Normally this shouldn't affect priv > system tools which are monitoring the same event as it shouldn't be > able to deplete resources as long as the resource control mechanisms > are configured and functioning properly; however, the memory usage > event puts all event listeners into a single contiguous table which a > !priv user can easily expand to a size where the table can no longer > be enlarged and if a priv system tool or another user tries to > register event afterwards, it'll fail. IOW, it creates a shared > resource which isn't properly provisioned and can be trivially filled > up making it an easy DoS target. > " > > Let's be more strict and cap the number of events that might be > registered. MAX_THRESHOLD_EVENTS value is more or less random. The > expectation is that it should be high enough to cover reasonable > usecases while not too high to allow excessive resources consumption. > 1024 events consume something like 16KB which shouldn't be a big deal > and it should be good enough. Is it correct that you fix one local DoS by introducing a new one? With the page the !priv user can block root from registering a threshold. Is it really the way we want to fix it? -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>