From patchwork Mon Jun 21 12:29:12 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Roberto Sassu X-Patchwork-Id: 12334701 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D064CC48BC2 for ; Mon, 21 Jun 2021 12:29:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B860160FF2 for ; Mon, 21 Jun 2021 12:29:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229710AbhFUMbr (ORCPT ); Mon, 21 Jun 2021 08:31:47 -0400 Received: from frasgout.his.huawei.com ([185.176.79.56]:3295 "EHLO frasgout.his.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229695AbhFUMbq (ORCPT ); Mon, 21 Jun 2021 08:31:46 -0400 Received: from fraeml714-chm.china.huawei.com (unknown [172.18.147.207]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4G7pQw6HZLz6H8DN; Mon, 21 Jun 2021 20:16:08 +0800 (CST) Received: from roberto-ThinkStation-P620.huawei.com (10.204.62.217) by fraeml714-chm.china.huawei.com (10.206.15.33) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Mon, 21 Jun 2021 14:29:29 +0200 From: Roberto Sassu To: , , , , CC: , , , , Roberto Sassu Subject: [PATCH v2] evm: Check xattr size discrepancy between kernel and user Date: Mon, 21 Jun 2021 14:29:12 +0200 Message-ID: <20210621122912.1472470-1-roberto.sassu@huawei.com> X-Mailer: git-send-email 2.25.1 MIME-Version: 1.0 X-Originating-IP: [10.204.62.217] X-ClientProxiedBy: lhreml752-chm.china.huawei.com (10.201.108.202) To fraeml714-chm.china.huawei.com (10.206.15.33) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: selinux@vger.kernel.org The kernel and the user obtain an xattr value in two different ways: kernel (EVM): uses vfs_getxattr_alloc() which obtains the xattr value from the filesystem handler (raw value); user (ima-evm-utils): uses vfs_getxattr() which obtains the xattr value from the LSMs (normalized value). Normally, this does not have an impact unless security.selinux is set with setfattr, with a value not terminated by '\0' (this is not the recommended way, security.selinux should be set with the appropriate tools such as chcon and restorecon). In this case, the kernel and the user see two different xattr values: the former sees the xattr value without '\0' (raw value), the latter sees the value with '\0' (value normalized by SELinux). This could result in two different verification outcomes from EVM and ima-evm-utils, if a signature was calculated with a security.selinux value terminated by '\0' and the value set in the filesystem is not terminated by '\0'. The former would report verification failure due to the missing '\0', while the latter would report verification success (because it gets the normalized value with '\0'). This patch mitigates this issue by comparing in evm_calc_hmac_or_hash() the size of the xattr returned by the two xattr functions and by warning the user if there is a discrepancy. Signed-off-by: Roberto Sassu Suggested-by: Mimi Zohar --- security/integrity/evm/evm_crypto.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/security/integrity/evm/evm_crypto.c b/security/integrity/evm/evm_crypto.c index 96b22f2ac27a..462c5258322a 100644 --- a/security/integrity/evm/evm_crypto.c +++ b/security/integrity/evm/evm_crypto.c @@ -221,7 +221,7 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry, size_t xattr_size = 0; char *xattr_value = NULL; int error; - int size; + int size, user_space_size; bool ima_present = false; if (!(inode->i_opflags & IOP_XATTR) || @@ -276,6 +276,12 @@ static int evm_calc_hmac_or_hash(struct dentry *dentry, if (size < 0) continue; + user_space_size = vfs_getxattr(&init_user_ns, dentry, + xattr->name, NULL, 0); + if (user_space_size != size) + pr_debug("file %s: xattr %s size mismatch (kernel: %d, user: %d)\n", + dentry->d_name.name, xattr->name, size, + user_space_size); error = 0; xattr_size = size; crypto_shash_update(desc, (const u8 *)xattr_value, xattr_size);