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Wed, 13 Jan 2021 21:11:13 +0000 (GMT) Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 74C4A42049; Wed, 13 Jan 2021 21:11:10 +0000 (GMT) Received: from li-f45666cc-3089-11b2-a85c-c57d1a57929f.ibm.com (unknown [9.160.57.196]) by d06av24.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 13 Jan 2021 21:11:10 +0000 (GMT) Message-ID: <71cddb6c8676ccd63c89364d805cfca76d32cb6e.camel@linux.ibm.com> Subject: Re: [PATCH v10 8/8] selinux: include a consumer of the new IMA critical data hook From: Mimi Zohar To: Paul Moore Cc: Tushar Sugandhi , Stephen Smalley , casey@schaufler-ca.com, agk@redhat.com, snitzer@redhat.com, gmazyland@gmail.com, tyhicks@linux.microsoft.com, sashal@kernel.org, James Morris , nramas@linux.microsoft.com, linux-integrity@vger.kernel.org, selinux@vger.kernel.org, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, dm-devel@redhat.com Date: Wed, 13 Jan 2021 16:11:09 -0500 In-Reply-To: References: <20210108040708.8389-1-tusharsu@linux.microsoft.com> <20210108040708.8389-9-tusharsu@linux.microsoft.com> <97328fc71687a0e1c327f6821548be9ba35bb193.camel@linux.ibm.com> Content-Type: text/plain; charset="ISO-8859-15" X-Mailer: Evolution 3.28.5 (3.28.5-14.el8) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.343,18.0.737 definitions=2021-01-13_11:2021-01-13,2021-01-13 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 priorityscore=1501 bulkscore=0 phishscore=0 mlxlogscore=999 spamscore=0 clxscore=1015 mlxscore=0 impostorscore=0 lowpriorityscore=0 malwarescore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2101130124 Precedence: bulk List-ID: X-Mailing-List: linux-integrity@vger.kernel.org On Wed, 2021-01-13 at 14:19 -0500, Paul Moore wrote: > On Wed, Jan 13, 2021 at 2:13 PM Mimi Zohar wrote: > > On Tue, 2021-01-12 at 11:27 -0500, Paul Moore wrote: > > > On Thu, Jan 7, 2021 at 11:07 PM Tushar Sugandhi > > > wrote: > > > > From: Lakshmi Ramasubramanian > > > > > > > > SELinux stores the active policy in memory, so the changes to this data > > > > at runtime would have an impact on the security guarantees provided > > > > by SELinux. Measuring in-memory SELinux policy through IMA subsystem > > > > provides a secure way for the attestation service to remotely validate > > > > the policy contents at runtime. > > > > > > > > Measure the hash of the loaded policy by calling the IMA hook > > > > ima_measure_critical_data(). Since the size of the loaded policy > > > > can be large (several MB), measure the hash of the policy instead of > > > > the entire policy to avoid bloating the IMA log entry. > > > > > > > > To enable SELinux data measurement, the following steps are required: > > > > > > > > 1, Add "ima_policy=critical_data" to the kernel command line arguments > > > > to enable measuring SELinux data at boot time. > > > > For example, > > > > BOOT_IMAGE=/boot/vmlinuz-5.10.0-rc1+ root=UUID=fd643309-a5d2-4ed3-b10d-3c579a5fab2f ro nomodeset security=selinux ima_policy=critical_data > > > > > > > > 2, Add the following rule to /etc/ima/ima-policy > > > > measure func=CRITICAL_DATA label=selinux > > > > > > > > Sample measurement of the hash of SELinux policy: > > > > > > > > To verify the measured data with the current SELinux policy run > > > > the following commands and verify the output hash values match. > > > > > > > > sha256sum /sys/fs/selinux/policy | cut -d' ' -f 1 > > > > > > > > grep "selinux-policy-hash" /sys/kernel/security/integrity/ima/ascii_runtime_measurements | tail -1 | cut -d' ' -f 6 > > > > > > > > Note that the actual verification of SELinux policy would require loading > > > > the expected policy into an identical kernel on a pristine/known-safe > > > > system and run the sha256sum /sys/kernel/selinux/policy there to get > > > > the expected hash. > > > > > > > > Signed-off-by: Lakshmi Ramasubramanian > > > > Suggested-by: Stephen Smalley > > > > Reviewed-by: Tyler Hicks > > > > --- > > > > Documentation/ABI/testing/ima_policy | 3 +- > > > > security/selinux/Makefile | 2 + > > > > security/selinux/ima.c | 64 ++++++++++++++++++++++++++++ > > > > security/selinux/include/ima.h | 24 +++++++++++ > > > > security/selinux/include/security.h | 3 +- > > > > security/selinux/ss/services.c | 64 ++++++++++++++++++++++++---- > > > > 6 files changed, 149 insertions(+), 11 deletions(-) > > > > create mode 100644 security/selinux/ima.c > > > > create mode 100644 security/selinux/include/ima.h > > > > > > I remain concerned about the possibility of bypassing a measurement by > > > tampering with the time, but I appear to be the only one who is > > > worried about this so I'm not going to block this patch on those > > > grounds. > > > > > > Acked-by: Paul Moore > > > > Thanks, Paul. > > > > Including any unique string would cause the buffer hash to change, > > forcing a new measurement. Perhaps they were concerned with > > overflowing a counter. > > My understanding is that Lakshmi wanted to force a new measurement > each time and felt using a timestamp would be the best way to do that. > A counter, even if it wraps, would have a different value each time > whereas a timestamp is vulnerable to time adjustments. While a > properly controlled and audited system could be configured and > monitored to detect such an event (I *think*), why rely on that if it > isn't necessary? Why are you saying that even if the counter wraps a new measurement is guaranteed. I agree with the rest of what you said. Mimi