From patchwork Wed Oct 23 23:39:46 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Lakshmi Ramasubramanian X-Patchwork-Id: 11208047 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 75F76139A for ; Wed, 23 Oct 2019 23:40:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4B5322084B for ; Wed, 23 Oct 2019 23:40:13 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="XVA1357l" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2392775AbfJWXkK (ORCPT ); Wed, 23 Oct 2019 19:40:10 -0400 Received: from linux.microsoft.com ([13.77.154.182]:51344 "EHLO linux.microsoft.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731522AbfJWXj6 (ORCPT ); Wed, 23 Oct 2019 19:39:58 -0400 Received: from nramas-ThinkStation-P520.corp.microsoft.com (unknown [131.107.174.108]) by linux.microsoft.com (Postfix) with ESMTPSA id B488820106BE; Wed, 23 Oct 2019 16:39:55 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com B488820106BE DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1571873995; bh=EbL/+S+i5zAm2sZAw2rfqgxrNxhU5pb2Dq78teEHmhs=; h=From:To:Cc:Subject:Date:From; b=XVA1357l0nZ26mmoPFX3oWFBAXzcOSzL64a8NqKvDGPpF9ArX1AWjqgmbvmyglMK1 akDj+Bzo1G92Ol1jvoVWTNBrroJsfC91cx4YdQ+cXEbtBcW2lTiUkEwskgRnTso9AN zgxWNsctsTREUD18jMsHdZAXxD7jOVqE8Wuja6ow= From: Lakshmi Ramasubramanian To: zohar@linux.ibm.com, dhowells@redhat.com, casey@schaufler-ca.com, sashal@kernel.org, jamorris@linux.microsoft.com, linux-security-module@vger.kernel.org, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, keyrings@vger.kernel.org Cc: nramas@linux.microsoft.com Subject: [PATCH v2 0/4] KEYS: measure keys when they are created or updated Date: Wed, 23 Oct 2019 16:39:46 -0700 Message-Id: <20191023233950.22072-1-nramas@linux.microsoft.com> X-Mailer: git-send-email 2.17.1 Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: Problem Statement: Keys created or updated in the system are currently not being measured. Therefore an attestation service, for instance, would not be able to attest whether or not the trusted keys keyring(s), for instance, contain only known good (trusted) keys. ima measures system files, command line arguments passed to kexec, and boot aggregate. It can be used to measure keys as well. But there is no mechanism available in the kernel for ima to know when a key is created or updated. This change aims to address measuring keys created or updated in the system. To achieve the above the following changes have been made: - Added a new ima hook namely, ima_post_key_create_or_update, which measures the key. The measurement can be controlled through ima policy. In this change set a new ima policy hook BUILTIN_TRUSTED_KEYS has been added to measure keys added to the builtin_trusted_keys keyring. In future, this can be extended to measure keys added to other keyrings. Change Log: v2: => Per suggestion from Mimi reordered the patch set to first enable measuring keys added or updated in the system. And, then scope the measurement to keys added to builtin_trusted_keys keyring through ima policy. => Removed security_key_create_or_update function and instead call ima hook, to measure the key, directly from key_create_or_update function. v1: => LSM function for key_create_or_update. It calls ima. => Added ima hook for measuring keys => ima measures keys based on ima policy. v0: => Added LSM hook for key_create_or_update. => Measure keys added to builtin or secondary trusted keys keyring. Background: Currently ima measures file hashes and .ima signatures. ima signatures are validated against keys in the ".ima" keyring. If the kernel is built with CONFIG_IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY enabled, then all keys in ".ima" keyring must be signed by a key in ".builtin_trusted_keys" or ".secondary_trusted_keys" keyrings. Although ima supports the above configuration, not having an insight into what keys are present in these trusted keys keyrings would prevent an attestation service from validating a client machine. On systems with CONFIG_IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY enabled, measuring keys in the ".builtin_trusted_keys" keyring provides a mechanism to attest that the client's system binaries are indeed signed by signers that chain to known trusted keys. Without this change, to attest the clients one needs to maintain an "allowed list" of file hashes of all versions of all client binaries that are deployed on the clients in the enterprise. That is a huge operational challenge in a large scale environment of clients with heterogenous builds. This also limits scalability and agility of rolling out frequent client binary updates. Testing performed: * Booted the kernel with this change. * Executed keyctl tests from the Linux Test Project (LTP) * Added a new key to a keyring and verified "key create" code path. => In this case added a key to builtin_trusted_keys keyring. => Verified ima measured this key only when a policy is set. * Added the same key again and verified "key update" code path. => Add the same key to builtin_trusted_keys keyring. => Verified ima measured the key only when a policy is set. Questions and concerns raised by reviewers on this patch set: Question 1: Is "Signed with a trusted key" equal to "Trusted file"? Doesn't the service need the hashes of the system files to determine whether a file is trusted or not? "Signed with a trusted key" does not equal "Trusted" Answer: Agree "Signed with a trusted key" may not equal "Trusted". To address this, the attesting service can maintain a small manageable set of bad hashes (a "Blocked list") and a list of trusted keys expected in client's .builtin_trusted_keys" keyring. Using this data, the service can detect the presence of "Disallowed (untrusted) version of client binaries". Question 2: Providing more data to the service (such as ".builtin_trusted_keys"), empowers the service to deny access to clients (block clients). IMA walks a fine line in enforcing and measuring file integrity. This patchset breaches that fine line and in doing so brings back the fears of trusted computing. Answer: Any new measurement we add in IMA will provide more data to service and can enable it to deny access to clients. It is not clear why this patch set would breach the fine line between measuring and enforcing. Since this patch set is disabled by default and enabled through CONFIG_IMA_MEASURE_TRUSTED_KEYS, only those enterprises that require this new measurement can opt-in for it. Since it is disabled by default, it does not restrict the autonomy of independent users who are unaffected by attestation. Question 3: IMA log already contains a pointer to the IMA keys used for signature verification. Why does the service need to care what keys were used to sign (install) the IMA keys? What is gained by measuring the keys in the ".builtin_trusted_keys" Answer: To attest the clients using the current IMA log, service needs to maintain hashes of all the deployed versions of all the system binaries for their enterprise. This will introduce a very high operational overhead in a large scale environment of clients with heterogenous builds. This limits scalability and agility of rolling out frequent client binary updates. On the other hand, with the current patch set, we will have IMA validate the file signature on the clients and the service validate that the IMA keys were installed using trusted keys. This provides a chain of trust: => IMA Key validates file signature on the client => Built-In trusted key attests IMA key on the client => Attestation service attests the Built-In trusted keys reported by the client in the IMA log This approach, therefore, would require the service to maintain a manageble set of trusted keys that it receives from a trusted source. And, verify if the clients only have keys from that set of trusted keys. Question 4: Where will the attestation service receive the keys to validate against? Answer: Attestation service will receive the keys from a trusted source such as the enterprise build services that provides the client builds. The service will use this set of keys to verify that the keys reported by the clients in the IMA log contains only keys from this trusted list. Question 5: What is changing in the IMA log through this patch set? Answer: This patch set does not remove any data that is currently included in the IMA log. It only adds more data to the IMA log - the data on ".builtin_trusted_keys" Lakshmi Ramasubramanian (4): KEYS: Defined an ima hook for measuring keys on key create or update KEYS: Queue key for measurement if ima is not initialized. Measure queued keys when ima is initialized KEYS: Added BUILTIN_TRUSTED_KEYS enum to measure keys added to builtin_trusted_keys keyring KEYS: Enabled ima policy to measure keys added to builtin_trusted_keys keyring Documentation/ABI/testing/ima_policy | 1 + certs/system_keyring.c | 5 ++ include/keys/system_keyring.h | 2 + include/linux/ima.h | 8 +++ security/integrity/ima/ima.h | 18 ++++++ security/integrity/ima/ima_api.c | 1 + security/integrity/ima/ima_init.c | 10 ++- security/integrity/ima/ima_main.c | 49 +++++++++++++++ security/integrity/ima/ima_policy.c | 5 +- security/integrity/ima/ima_queue.c | 94 ++++++++++++++++++++++++++++ security/keys/key.c | 9 +++ 11 files changed, 200 insertions(+), 2 deletions(-)