Commit fdd1ffe8 authored by Lakshmi Ramasubramanian's avatar Lakshmi Ramasubramanian Committed by Mimi Zohar
Browse files

selinux: include a consumer of the new IMA critical data hook 



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: default avatarLakshmi Ramasubramanian <nramas@linux.microsoft.com>
Suggested-by: default avatarStephen Smalley <stephen.smalley.work@gmail.com>
Acked-by: default avatarPaul Moore <paul@paul-moore.com>
Reviewed-by: default avatarTyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: default avatarMimi Zohar <zohar@linux.ibm.com>
parent 03cee168

Documentation/ABI/testing/ima_policy

+2 −1
Original line number Diff line number Diff line
@@ -52,8 +52,9 @@ Description: 
			template:= name of a defined IMA template type

			(eg, ima-ng). Only valid when action is "measure".

			pcr:= decimal value

			label:= [data_label]

			label:= [selinux]|[data_label]

			data_label:= a unique string used for grouping and limiting critical data.

			For example, "selinux" to measure critical data for SELinux.



		  default policy:

			# PROC_SUPER_MAGIC

security/selinux/Makefile

+2 −0
Original line number Diff line number Diff line
@@ -16,6 +16,8 @@ selinux-$(CONFIG_NETLABEL) += netlabel.o 


selinux-$(CONFIG_SECURITY_INFINIBAND) += ibpkey.o



selinux-$(CONFIG_IMA) += ima.o



ccflags-y := -I$(srctree)/security/selinux -I$(srctree)/security/selinux/include



$(addprefix $(obj)/,$(selinux-y)): $(obj)/flask.h

security/selinux/ima.c

0 → 100644
+44 −0
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0+

/*

 * Copyright (C) 2021 Microsoft Corporation

 *

 * Author: Lakshmi Ramasubramanian (nramas@linux.microsoft.com)

 *

 * Measure critical data structures maintainted by SELinux

 * using IMA subsystem.

 */

#include <linux/vmalloc.h>

#include <linux/ima.h>

#include "security.h"

#include "ima.h"



/*

 * 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)

{

	void *policy = NULL;

	size_t policy_len;

	int rc = 0;



	/*

	 * Measure SELinux policy only after initialization is completed.

	 */

	if (!selinux_initialized(state))

		return;



	rc = security_read_state_kernel(state, &policy, &policy_len);

	if (rc) {

		pr_err("SELinux: %s: failed to read policy %d.\n", __func__, rc);

		return;

	}



	ima_measure_critical_data("selinux", "selinux-policy-hash",

				  policy, policy_len, true);



	vfree(policy);

}

security/selinux/include/ima.h

0 → 100644
+24 −0
Original line number Diff line number Diff line 
/* SPDX-License-Identifier: GPL-2.0+ */

/*

 * Copyright (C) 2021 Microsoft Corporation

 *

 * Author: Lakshmi Ramasubramanian (nramas@linux.microsoft.com)

 *

 * Measure critical data structures maintainted by SELinux

 * using IMA subsystem.

 */



#ifndef _SELINUX_IMA_H_

#define _SELINUX_IMA_H_



#include "security.h"



#ifdef CONFIG_IMA

extern void selinux_ima_measure_state(struct selinux_state *selinux_state);

#else

static inline void selinux_ima_measure_state(struct selinux_state *selinux_state)

{

}

#endif



#endif	/* _SELINUX_IMA_H_ */

security/selinux/include/security.h

+2 −1
Original line number Diff line number Diff line
@@ -229,7 +229,8 @@ void selinux_policy_cancel(struct selinux_state *state, 
			struct selinux_policy *policy);

int security_read_policy(struct selinux_state *state,

			 void **data, size_t *len);



int security_read_state_kernel(struct selinux_state *state,

			       void **data, size_t *len);

int security_policycap_supported(struct selinux_state *state,

				 unsigned int req_cap);