From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id A8253EC8742 for ; Thu, 7 Sep 2023 17:25:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234340AbjIGRZl convert rfc822-to-8bit (ORCPT ); Thu, 7 Sep 2023 13:25:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39084 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238248AbjIGRZd (ORCPT ); Thu, 7 Sep 2023 13:25:33 -0400 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.86.151]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5F13EE7F for ; Thu, 7 Sep 2023 10:25:07 -0700 (PDT) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-142-mAsshBkwN4ycognTopktyQ-1; Thu, 07 Sep 2023 09:55:54 +0100 X-MC-Unique: mAsshBkwN4ycognTopktyQ-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Thu, 7 Sep 2023 09:55:51 +0100 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Thu, 7 Sep 2023 09:55:51 +0100 From: David Laight To: 'Kees Cook' CC: "linux-kernel@vger.kernel.org" , "'linux-mm@kvack.org'" , 'Vlastimil Babka' , 'Christoph Lameter' , 'Pekka Enberg' , 'David Rientjes' , 'Joonsoo Kim' , 'Andrew Morton' , "'Eric Dumazet'" Subject: RE: [PATCH] slab: kmalloc_size_roundup() must not return 0 for non-zero size Thread-Topic: [PATCH] slab: kmalloc_size_roundup() must not return 0 for non-zero size Thread-Index: AdngmrOo1jL765q1RK2Ib5JvSvHtFwASyysAACBZzlA= Date: Thu, 7 Sep 2023 08:55:51 +0000 Message-ID: References: <202309061106.C0690BDBB@keescook> In-Reply-To: <202309061106.C0690BDBB@keescook> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Kees Cook > Sent: 06 September 2023 19:17 > > On Wed, Sep 06, 2023 at 08:18:21AM +0000, David Laight wrote: > > The typical use of kmalloc_size_roundup() is: > > ptr = kmalloc(sz = kmalloc_size_roundup(size), ...); > > if (!ptr) return -ENOMEM. > > This means it is vitally important that the returned value isn't > > less than the argument even if the argument is insane. > > In particular if kmalloc_slab() fails or the value is above > > (MAX_ULONG - PAGE_SIZE) zero is returned and kmalloc() will return > > it's single zero-length buffer. > > > > Fix by returning the input size on error or if the size exceeds > > a 'sanity' limit. > > kmalloc() will then return NULL is the size really is too big. > > > > Signed-off-by: David Laight > > Fixes: 05a940656e1eb ("slab: Introduce kmalloc_size_roundup()") > > --- > > The 'sanity limit' value doesn't really matter (even if too small) > > It could be 'MAX_ORDER + PAGE_SHIFT' but one ppc64 has MAX_ORDER 16 > > and I don't know if that also has large pages. > > Maybe it could be 1ul << 30 on 64bit, but it really doesn't matter > > if it is too big. > > I agree that returning 0 for an (impossible to reach) non-zero > is wrong, but the problem seen in netdev was that a truncation happened > for a value returned by kmalloc_size_roundup(). > > So, for the first, it shouldn't be possible for "c" to ever be NULL here: If it isn't possible there is no need to check :-) > > c = kmalloc_slab(size, GFP_KERNEL, 0); > return c ? c->object_size : 0; > > But sure, we can return KMALLOC_MAX_SIZE for that. Isn't KMALLOC_MAX_SIZE actually valid? - so would be wrong. Returning 'size' is always valid, the later kmalloc() will almost certainly fail, but it is also ok if it suceeds. > The pathological case was this: s/pathological/failing/ > > unsigned int truncated; > size_t fullsize = UINT_MAX + 1; > > ptr = kmalloc(truncated = kmalloc_size_roundup(fullsize), ...); The actual pathological case is: kmalloc(kmalloc_size_roundup(~0ULL - PAGESIZE/2), ...) which is kmalloc(0, ...) and suceeds. > Should the logic be changed to return KMALLOC_MAX_SIZE for anything > larger than KMALLOC_MAX_SIZE? This seems like a different kind of > foot-gun. > > Everything else in the allocator sanity checking (e.g. struct_size(), > etc) uses SIZE_MAX as the saturation value, which is why > kmalloc_size_roundup() did too. SIZE_MAX (aka ~0ull) seems far too large for sanity checking lengths. (Even without the issue of having no headroom.) A limit related to an upper bound for vmalloc() would probably be more appropriate, or maybe just a limit based on kernel VA. So for 32bit 2^30 is way too large for any kind of allocate. For 64bit you can go higher (even if the allocators can't support the values), maybe 2^48? David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)