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Wed, 13 Jan 2021 19:13:10 +0000 (GMT) Received: from li-f45666cc-3089-11b2-a85c-c57d1a57929f.ibm.com (unknown [9.160.57.196]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 13 Jan 2021 19:13:10 +0000 (GMT) Message-ID: <97328fc71687a0e1c327f6821548be9ba35bb193.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 , Tushar Sugandhi Cc: 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 14:13:09 -0500 In-Reply-To: References: <20210108040708.8389-1-tusharsu@linux.microsoft.com> <20210108040708.8389-9-tusharsu@linux.microsoft.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_09:2021-01-13,2021-01-13 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 malwarescore=0 impostorscore=0 mlxscore=0 phishscore=0 bulkscore=0 adultscore=0 priorityscore=1501 mlxlogscore=999 clxscore=1015 spamscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2009150000 definitions=main-2101130113 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Mimi > > +/* > > + * selinux_ima_measure_state - Measure hash of the SELinux policy > > + * > > + * @state: selinux state struct > > + * > > + * NOTE: This function must be called with policy_mutex held. > > + */ > > +void selinux_ima_measure_state(struct selinux_state *state) > > +{ > > + struct timespec64 cur_time; > > + void *policy = NULL; > > + char *policy_event_name = NULL; > > + size_t policy_len; > > + int rc = 0; > > + > > + /* > > + * Measure SELinux policy only after initialization is completed. > > + */ > > + if (!selinux_initialized(state)) > > + return; > > + > > + /* > > + * Pass a unique "event_name" to the IMA hook so that IMA subsystem > > + * will always measure the given data. > > + */ > > + ktime_get_real_ts64(&cur_time); > > + policy_event_name = kasprintf(GFP_KERNEL, "%s-%lld:%09ld", > > + "selinux-policy-hash", > > + cur_time.tv_sec, cur_time.tv_nsec); > > + if (!policy_event_name) { > > + pr_err("SELinux: %s: event name for policy not allocated.\n", > > + __func__); > > + goto out; > > + } > > +