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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 <nramas@linux.microsoft.com> Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com> Acked-by: Paul Moore <paul@paul-moore.com> Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com> Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
45 lines
1012 B
C
45 lines
1012 B
C
// SPDX-License-Identifier: GPL-2.0+
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/*
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* Copyright (C) 2021 Microsoft Corporation
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*
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* Author: Lakshmi Ramasubramanian (nramas@linux.microsoft.com)
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*
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* Measure critical data structures maintainted by SELinux
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* using IMA subsystem.
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*/
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#include <linux/vmalloc.h>
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#include <linux/ima.h>
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#include "security.h"
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#include "ima.h"
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/*
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* selinux_ima_measure_state - Measure hash of the SELinux policy
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*
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* @state: selinux state struct
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*
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* NOTE: This function must be called with policy_mutex held.
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*/
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void selinux_ima_measure_state(struct selinux_state *state)
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{
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void *policy = NULL;
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size_t policy_len;
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int rc = 0;
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/*
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* Measure SELinux policy only after initialization is completed.
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*/
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if (!selinux_initialized(state))
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return;
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rc = security_read_state_kernel(state, &policy, &policy_len);
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if (rc) {
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pr_err("SELinux: %s: failed to read policy %d.\n", __func__, rc);
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return;
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}
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ima_measure_critical_data("selinux", "selinux-policy-hash",
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policy, policy_len, true);
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vfree(policy);
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}
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