Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751466AbdGQTLp (ORCPT ); Mon, 17 Jul 2017 15:11:45 -0400 Received: from mail-it0-f49.google.com ([209.85.214.49]:35259 "EHLO mail-it0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751315AbdGQTLn (ORCPT ); Mon, 17 Jul 2017 15:11:43 -0400 MIME-Version: 1.0 In-Reply-To: References: <1500309907-9357-1-git-send-email-alex.popov@linux.com> <20170717175459.GC14983@bombadil.infradead.org> From: Kees Cook Date: Mon, 17 Jul 2017 12:11:40 -0700 X-Google-Sender-Auth: 42MT8KDZYHHIC7NBP5hfmO7Qaoc Message-ID: Subject: Re: [PATCH 1/1] mm/slub.c: add a naive detection of double free or corruption To: Alexander Popov Cc: Christopher Lameter , Matthew Wilcox , Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , Linux-MM , LKML , "kernel-hardening@lists.openwall.com" Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2150 Lines: 54 On Mon, Jul 17, 2017 at 12:01 PM, Alexander Popov wrote: > Hello Christopher, > > Thanks for your reply. > > On 17.07.2017 21:04, Christopher Lameter wrote: >> On Mon, 17 Jul 2017, Matthew Wilcox wrote: >> >>> On Mon, Jul 17, 2017 at 07:45:07PM +0300, Alexander Popov wrote: >>>> Add an assertion similar to "fasttop" check in GNU C Library allocator: >>>> an object added to a singly linked freelist should not point to itself. >>>> That helps to detect some double free errors (e.g. CVE-2017-2636) without >>>> slub_debug and KASAN. Testing with hackbench doesn't show any noticeable >>>> performance penalty. >>> >>>> { >>>> + BUG_ON(object == fp); /* naive detection of double free or corruption */ >>>> *(void **)(object + s->offset) = fp; >>>> } >>> >>> Is BUG() the best response to this situation? If it's a corruption, then >>> yes, but if we spot a double-free, then surely we should WARN() and return >>> without doing anything? >> >> The double free debug checking already does the same thing in a more >> thourough way (this one only checks if the last free was the same >> address). So its duplicating a check that already exists. > > Yes, absolutely. Enabled slub_debug (or KASAN with its quarantine) can detect > more double-free errors. But it introduces much bigger performance penalty and > it's disabled by default. > >> However, this one is always on. > > Yes, I would propose to have this relatively cheap check enabled by default. I > think it will block a good share of double-free errors. Currently it's really > easy to turn such a double-free into use-after-free and exploit it, since, as I > wrote, next two kmalloc() calls return the same address. So we could make > exploiting harder for a relatively low price. > > Christopher, if I change BUG_ON() to VM_BUG_ON(), it will be disabled by default > again, right? Let's merge this with the proposed CONFIG_FREELIST_HARDENED, then the performance change is behind a config, and we gain the rest of the freelist protections at the same time: http://www.openwall.com/lists/kernel-hardening/2017/07/06/1 -Kees -- Kees Cook Pixel Security