Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751410AbdGQTB2 (ORCPT ); Mon, 17 Jul 2017 15:01:28 -0400 Received: from mail-lf0-f67.google.com ([209.85.215.67]:34149 "EHLO mail-lf0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751320AbdGQTB1 (ORCPT ); Mon, 17 Jul 2017 15:01:27 -0400 Reply-To: alex.popov@linux.com Subject: Re: [PATCH 1/1] mm/slub.c: add a naive detection of double free or corruption To: Christopher Lameter , Matthew Wilcox Cc: Pekka Enberg , David Rientjes , Joonsoo Kim , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com, keescook@chromium.org References: <1500309907-9357-1-git-send-email-alex.popov@linux.com> <20170717175459.GC14983@bombadil.infradead.org> From: Alexander Popov Message-ID: Date: Mon, 17 Jul 2017 22:01:15 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1764 Lines: 44 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? Best regards, Alexander