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[2620:137:e000::1:18]) by mx.google.com with ESMTPS id w5-20020a17090abc0500b002634d2ab01bsi5373579pjr.130.2023.08.19.08.45.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 19 Aug 2023 08:45:24 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20221208 header.b=NHuGuvQV; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id D1DE3EF97B; Sat, 19 Aug 2023 01:48:24 -0700 (PDT) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1379411AbjHRSSv (ORCPT + 99 others); Fri, 18 Aug 2023 14:18:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39166 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1379467AbjHRSSj (ORCPT ); Fri, 18 Aug 2023 14:18:39 -0400 Received: from mail-wm1-x32e.google.com (mail-wm1-x32e.google.com [IPv6:2a00:1450:4864:20::32e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 24CDD2D72 for ; Fri, 18 Aug 2023 11:18:34 -0700 (PDT) Received: by mail-wm1-x32e.google.com with SMTP id 5b1f17b1804b1-3fe2a116565so9435e9.1 for ; Fri, 18 Aug 2023 11:18:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1692382712; x=1692987512; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=h3j4Mqn/PbeNoccarhbs+EdB7Fu2ck9H3LC4u5W/k/o=; b=NHuGuvQVCXo1QS1pE+nZc/9rYhs+1IJwmwoSCY6rdKHSK61HzJrCDuO7aEL5x4uNwL a7A6bHEhYKjE5qjg9dTUsTTrtMBmzrlVo/MTjNdAb/S6l3ovkXRFpb+fTlbV6woR9G5l GSZzg7HdUJTo7VHEkeXUvGIvP+15UKjOILSYNKGaTmWuXI/mQjYVSZ7t8ZJxS2KZIg4z gHu+D091lYezJYa48+Xh+k7boUNygg2PFF0D5rbLQ/wUoUp2jpC0opS3S6ZXMLTqL1uY AQtgF5sL8zAjxdxNO6M93RrFv/vlGa5Kc1I6wyKTJZN+X/nnBGKfv8VtB+ECbiEyrhvA H2rw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1692382712; x=1692987512; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=h3j4Mqn/PbeNoccarhbs+EdB7Fu2ck9H3LC4u5W/k/o=; b=I2K0q95vbmumKfDjEX7gxTlix6KEAauCBoG2w/7NK2V3+D3RZytLUGx/3szdblrofR J9de5YsHESOtfbAmuTW0HovQLti6jFQVU9mDzawS6xCnb8SgxdJJNJX01bRtgrWbAyFa hk3HpElpA9mq+q9184FIOBxJDh61qhDWqwn0UcTtRyIOb8hkuRBB1QpRF0X2uJWGS7dQ 9AlLfNHFy4BFGSJCp/4C7aL9aKG2QcEX2duaLeM44WzwG4IbS03Ti6klT0ObbwbH+q6B PXUltLtONQsEOW32yr/utFq89zyzrhRSLeBKbNf4GJLIkx+E9i65bB1n+RwbRl36vLvK FHJA== X-Gm-Message-State: AOJu0Ywn5Oz/txlMobA3a/menZFAbiabWypAgWMdyeuuUL8/vEiVA0s8 ltGQIqjJb+sHt7BHxCTeg7/Uwqh682aS5obAERq2fw== X-Received: by 2002:a05:600c:1907:b0:3f4:fb7:48d4 with SMTP id j7-20020a05600c190700b003f40fb748d4mr106532wmq.3.1692382712518; Fri, 18 Aug 2023 11:18:32 -0700 (PDT) MIME-Version: 1.0 References: <20230818041740.gonna.513-kees@kernel.org> <20230818105542.a6b7c41c47d4c6b9ff2e8839@linux-foundation.org> In-Reply-To: <20230818105542.a6b7c41c47d4c6b9ff2e8839@linux-foundation.org> From: Jann Horn Date: Fri, 18 Aug 2023 20:17:55 +0200 Message-ID: Subject: Re: [PATCH v2] creds: Convert cred.usage to refcount_t To: Andrew Morton Cc: Kees Cook , linux-hardening@vger.kernel.org, Elena Reshetova , David Windsor , Hans Liljestrand , Trond Myklebust , Anna Schumaker , Chuck Lever , Jeff Layton , Neil Brown , Olga Kornievskaia , Dai Ngo , Tom Talpey , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Sergey Senozhatsky , Alexey Gladkov , "Eric W. Biederman" , Yu Zhao , linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, netdev@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Spam-Status: No, score=-17.6 required=5.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, ENV_AND_HDR_SPF_MATCH,RCVD_IN_DNSWL_BLOCKED,SPF_HELO_NONE,SPF_PASS, USER_IN_DEF_DKIM_WL,USER_IN_DEF_SPF_WL autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Aug 18, 2023 at 7:56=E2=80=AFPM Andrew Morton wrote: > On Thu, 17 Aug 2023 21:17:41 -0700 Kees Cook wrot= e: > > > From: Elena Reshetova > > > > atomic_t variables are currently used to implement reference counters > > with the following properties: > > - counter is initialized to 1 using atomic_set() > > - a resource is freed upon counter reaching zero > > - once counter reaches zero, its further > > increments aren't allowed > > - counter schema uses basic atomic operations > > (set, inc, inc_not_zero, dec_and_test, etc.) > > > > Such atomic variables should be converted to a newly provided > > refcount_t type and API that prevents accidental counter overflows and > > underflows. This is important since overflows and underflows can lead > > to use-after-free situation and be exploitable. > > ie, if we have bugs which we have no reason to believe presently exist, > let's bloat and slow down the kernel just in case we add some in the > future? Yeah. Or in case we currently have some that we missed. Though really we don't *just* need refcount_t to catch bugs; on a system with enough RAM you can also overflow many 32-bit refcounts by simply creating 2^32 actual references to an object. Depending on the structure of objects that hold such refcounts, that can start happening at around 2^32 * 8 bytes =3D 32 GiB memory usage, and it becomes increasingly practical to do this with more objects if you have significantly more RAM. I suppose you could avoid such issues by putting a hard limit of 32 GiB on the amount of slab memory and requiring that kernel object references are stored as pointers in slab memory, or by making all the refcounts 64-bit.