Re: [PATCH v10 8/8] selinux: include a consumer of the new IMA critical data hook

Paul Moore
2021-01-12T16:28:15Z

On Thu, Jan 7, 2021 at 11:07 PM Tushar Sugandhi
<tusharsu@linux.microsoft.com> wrote:
> From: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
>
> 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>
> Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> ---
> 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 <paul@paul-moore.com>

> diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
> index 54fe1c15ed50..8365596cb42b 100644
> --- a/Documentation/ABI/testing/ima_policy
> +++ b/Documentation/ABI/testing/ima_policy
> @@ -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
> diff --git a/security/selinux/Makefile b/security/selinux/Makefile
> index 4d8e0e8adf0b..776162444882 100644
> --- a/security/selinux/Makefile
> +++ b/security/selinux/Makefile
> @@ -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
> diff --git a/security/selinux/ima.c b/security/selinux/ima.c
> new file mode 100644
> index 000000000000..0b835bdc3aa9
> --- /dev/null
> +++ b/security/selinux/ima.c
> @@ -0,0 +1,64 @@
> +// 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/ktime.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)
> +{
> + 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;
> + }
> +
> + rc = security_read_state_kernel(state, &policy, &policy_len);
> + if (rc) {
> + pr_err("SELinux: %s: failed to read policy %d.\n", __func__, rc);
> + goto out;
> + }
> +
> + ima_measure_critical_data("selinux", policy_event_name,
> + policy, policy_len, true);
> +
> + vfree(policy);
> +
> +out:
> + kfree(policy_event_name);
> +}
> diff --git a/security/selinux/include/ima.h b/security/selinux/include/ima.h
> new file mode 100644
> index 000000000000..d69c36611423
> --- /dev/null
> +++ b/security/selinux/include/ima.h
> @@ -0,0 +1,24 @@
> +/* 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_ */
> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> index 3cc8bab31ea8..29cae32d3fc5 100644
> --- a/security/selinux/include/security.h
> +++ b/security/selinux/include/security.h
> @@ -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);
>
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index 9704c8a32303..cc8dbc4ed8db 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -65,6 +65,7 @@
> #include "ebitmap.h"
> #include "audit.h"
> #include "policycap_names.h"
> +#include "ima.h"
>
> /* Forward declaration. */
> static int context_struct_to_string(struct policydb *policydb,
> @@ -2180,6 +2181,7 @@ static void selinux_notify_policy_change(struct selinux_state *state,
> selinux_status_update_policyload(state, seqno);
> selinux_netlbl_cache_invalidate();
> selinux_xfrm_notify_policyload();
> + selinux_ima_measure_state(state);
> }
>
> void selinux_policy_commit(struct selinux_state *state,
> @@ -3875,8 +3877,33 @@ int security_netlbl_sid_to_secattr(struct selinux_state *state,
> }
> #endif /* CONFIG_NETLABEL */
>
> +/**
> + * __security_read_policy - read the policy.
> + * @policy: SELinux policy
> + * @data: binary policy data
> + * @len: length of data in bytes
> + *
> + */
> +static int __security_read_policy(struct selinux_policy *policy,
> + void *data, size_t *len)
> +{
> + int rc;
> + struct policy_file fp;
> +
> + fp.data = data;
> + fp.len = *len;
> +
> + rc = policydb_write(&policy->policydb, &fp);
> + if (rc)
> + return rc;
> +
> + *len = (unsigned long)fp.data - (unsigned long)data;
> + return 0;
> +}
> +
> /**
> * security_read_policy - read the policy.
> + * @state: selinux_state
> * @data: binary policy data
> * @len: length of data in bytes
> *
> @@ -3885,8 +3912,6 @@ int security_read_policy(struct selinux_state *state,
> void **data, size_t *len)
> {
> struct selinux_policy *policy;
> - int rc;
> - struct policy_file fp;
>
> policy = rcu_dereference_protected(
> state->policy, lockdep_is_held(&state->policy_mutex));
> @@ -3898,14 +3923,35 @@ int security_read_policy(struct selinux_state *state,
> if (!*data)
> return -ENOMEM;
>
> - fp.data = *data;
> - fp.len = *len;
> + return __security_read_policy(policy, *data, len);
> +}
>
> - rc = policydb_write(&policy->policydb, &fp);
> - if (rc)
> - return rc;
> +/**
> + * security_read_state_kernel - read the policy.
> + * @state: selinux_state
> + * @data: binary policy data
> + * @len: length of data in bytes
> + *
> + * Allocates kernel memory for reading SELinux policy.
> + * This function is for internal use only and should not
> + * be used for returning data to user space.
> + *
> + * This function must be called with policy_mutex held.
> + */
> +int security_read_state_kernel(struct selinux_state *state,
> + void **data, size_t *len)
> +{
> + struct selinux_policy *policy;
>
> - *len = (unsigned long)fp.data - (unsigned long)*data;
> - return 0;
> + policy = rcu_dereference_protected(
> + state->policy, lockdep_is_held(&state->policy_mutex));
> + if (!policy)
> + return -EINVAL;
> +
> + *len = policy->policydb.len;
> + *data = vmalloc(*len);
> + if (!*data)
> + return -ENOMEM;
>
> + return __security_read_policy(policy, *data, len);
> }
> --
> 2.17.1
>

--
paul moore