From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753557AbbIBXIN (ORCPT ); Wed, 2 Sep 2015 19:08:13 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:37241 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752464AbbIBXIL (ORCPT ); Wed, 2 Sep 2015 19:08:11 -0400 Date: Thu, 3 Sep 2015 01:08:04 +0200 From: Peter Zijlstra To: Vince Weaver Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Arnaldo Carvalho de Melo Subject: Re: perf: bug, kernel ignores the buffer size on large read Message-ID: <20150902230804.GX19282@twins.programming.kicks-ass.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 02, 2015 at 12:34:41PM -0400, Vince Weaver wrote: > > OK, this time I found the actual bug. > > event->read_size is declared as a u16 in include/linux/perf_event.h > > but it is very easy to get event->read_size larger than 64k > (in my case, create 10000 events in a group). > > Because we wrap around the u16, the > if (count < event->read_size) return -ENOSPC; > in perf_read_hw() doesn't trigger reliably and so if you do a read > on a large group event the kernel will quite happily copy_to_user() > beyond the bounds of the value set in the read syscall. > > In my case it completely smashed the stack and caused the program to > segfault. > > I'm not sure what the solution is here. Change read_size to be larger? > Ban events whose read size would be larger than 64k? Although that gets > tricky because the related header_size is also only a u16. I think we should try and just ban events with a read_size > 64k; that's one _large_ group -- there's other issues with that as well I imagine. If we really want to allow something larger, we _could_ fudge something for cases where we do not have PERF_SAMPLE_READ set, but that's not particularly nice either.