Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752713Ab1CFA5j (ORCPT ); Sat, 5 Mar 2011 19:57:39 -0500 Received: from mx1.vsecurity.com ([209.67.252.12]:64523 "EHLO mx1.vsecurity.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752065Ab1CFA5i (ORCPT ); Sat, 5 Mar 2011 19:57:38 -0500 Subject: Re: [PATCH] Make /proc/slabinfo 0400 From: Dan Rosenberg To: Jesper Juhl Cc: Pekka Enberg , Matt Mackall , Linus Torvalds , Dave Hansen , Theodore Tso , cl@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ingo Molnar , Andrew Morton In-Reply-To: References: <1299174652.2071.12.camel@dan> <1299185882.3062.233.camel@calx> <1299186986.2071.90.camel@dan> <1299188667.3062.259.camel@calx> <1299191400.2071.203.camel@dan> <2DD7330B-2FED-4E58-A76D-93794A877A00@mit.edu> <1299260164.8493.4071.camel@nimitz> <1299262495.3062.298.camel@calx> <1299271041.2071.1398.camel@dan> <1299273034.2071.1417.camel@dan> Content-Type: text/plain; charset="UTF-8" Date: Sat, 05 Mar 2011 19:57:32 -0500 Message-ID: <1299373052.2071.1431.camel@dan> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2896 Lines: 56 On Sun, 2011-03-06 at 01:42 +0100, Jesper Juhl wrote: > On Fri, 4 Mar 2011, Dan Rosenberg wrote: > > > On Fri, 2011-03-04 at 22:58 +0200, Pekka Enberg wrote: > > > On Fri, Mar 4, 2011 at 10:37 PM, Dan Rosenberg wrote: > > > > This patch makes these techniques more difficult by making it hard to > > > > know whether the last attacker-allocated object resides before a free or > > > > allocated object. Especially with vulnerabilities that only allow one > > > > attempt at exploitation before recovery is needed to avoid trashing too > > > > much heap state and causing a crash, this could go a long way. I'd > > > > still argue in favor of removing the ability to know how many objects > > > > are used in a given slab, since randomizing objects doesn't help if you > > > > know every object is allocated. > > > > > > So if the attacker knows every object is allocated, how does that help > > > if we're randomizing the initial freelist? > > > > If you know you've got a slab completely full of your objects, then it > > doesn't matter that they happened to be allocated in a random fashion - > > they're still all allocated, and by freeing one of them and > > reallocating, you'll still be next to your target. > > > > But still, if randomizing allocations makes life just a little harder for > attackers in some scenarios, why not just do it? > Same with making /proc/slabinfo 0400, if it just makes things a little > harder in a few cases, why not do it? It's not like a admin who needs > /proc/slabinfo to have other permissions can't arrange for that. > > Having been employed as a systems administrator for many years and having > seen many a box cracked, my oppinion is that every little bit helps. The > kernel is currently not a hard target and everything we can do to harden > it is a good thing (within reason of course). > > Why not just do both randomization and 0400 as a start? We can always > harden further later. I agree that there's no harm in these patches, and they might make it (only) slightly harder in some cases, so yes, we might as well. I just don't want to trick anyone into a false sense of security by thinking that these measures by themselves are doing anything especially substantial to prevent heap exploits. But as you say, it's a start. Another hardening measure that's been mentioned before is validating the address of each to-be-returned pointer during allocation, to avoid attacks that rely on corrupting free list pointers (i.e. compare against TASK_SIZE). But then we're talking about introducing additional overhead into every single kmalloc() call. -Dan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/