[PATCH v7 8/8] selinux: include a consumer of the new IMA critical data hook
Tyler Hicks
tyhicks at linux.microsoft.com
Fri Dec 11 15:36:18 UTC 2020
On 2020-12-09 11:42:12, Tushar Sugandhi wrote:
> From: Lakshmi Ramasubramanian <nramas at linux.microsoft.com>
>
> IMA measures files and buffer data such as keys, command line arguments
> passed to the kernel on kexec system call, etc. While these measurements
> enable monitoring and validating the integrity of the system, it is not
> sufficient. Various data structures, policies and states stored in kernel
> memory also impact the integrity of the system. Updates to these data
> structures would have an impact on the security functionalities.
This is repetitive when looking at the entire series. I think it can be
dropped.
> For example, SELinux stores the active policy in memory. Changes to this
Start here and drop the "For example, ":
SELinux stores the active policy in memory and changes to this data ...
> data at runtime would have an impact on the security guarantees provided
> by SELinux. Measuring such in-memory data structures through IMA
> subsystem provides a secure way for a remote attestation service to
> know the state of the system and also the runtime changes in the state
> of the system.
>
> SELinux policy is a critical data for this security module that needs
SELinux policy is critical data and should be measured. This measurement ...
> to be measured. This measurement can be used by an attestation service,
> for instance, to verify if the policy has been setup correctly and that
> it hasn't been tampered at run-time.
>
> 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.
>
> Add "selinux" to the list of supported data sources maintained by IMA
> to enable measuring SELinux data.
>
> 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 data_source=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 at linux.microsoft.com>
> Suggested-by: Stephen Smalley <stephen.smalley.work at gmail.com>
> ---
> Documentation/ABI/testing/ima_policy | 3 +-
> security/selinux/Makefile | 2 +
> security/selinux/include/security.h | 11 +++-
> security/selinux/measure.c | 86 ++++++++++++++++++++++++++++
> security/selinux/ss/services.c | 71 ++++++++++++++++++++---
> 5 files changed, 162 insertions(+), 11 deletions(-)
> create mode 100644 security/selinux/measure.c
>
> diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
> index 0f4ee9e0a455..7c7023f7986b 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
> - data_source:= [label]
> + data_source:= [selinux]|[label]
> 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..83d512116341 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-$(CONFIG_IMA) += measure.o
> +
> ccflags-y := -I$(srctree)/security/selinux -I$(srctree)/security/selinux/include
>
> $(addprefix $(obj)/,$(selinux-y)): $(obj)/flask.h
> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> index 3cc8bab31ea8..18ee65c98446 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_policy_kernel(struct selinux_state *state,
> + void **data, size_t *len);
> int security_policycap_supported(struct selinux_state *state,
> unsigned int req_cap);
>
> @@ -446,4 +447,12 @@ extern void ebitmap_cache_init(void);
> extern void hashtab_cache_init(void);
> extern int security_sidtab_hash_stats(struct selinux_state *state, char *page);
>
> +#ifdef CONFIG_IMA
> +extern void selinux_measure_state(struct selinux_state *selinux_state);
> +#else
> +static inline void selinux_measure_state(struct selinux_state *selinux_state)
> +{
> +}
> +#endif
> +
> #endif /* _SELINUX_SECURITY_H_ */
> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
> new file mode 100644
> index 000000000000..c409ada6ea39
> --- /dev/null
> +++ b/security/selinux/measure.c
> @@ -0,0 +1,86 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Measure SELinux state using IMA subsystem.
> + */
> +#include <linux/vmalloc.h>
> +#include <linux/ktime.h>
> +#include <linux/ima.h>
> +#include "security.h"
> +
> +/*
> + * This function creates a unique name by appending the timestamp to
> + * the given string. This string is passed as "event_name" to the IMA
> + * hook to measure the given SELinux data.
> + *
> + * The data provided by SELinux to the IMA subsystem for measuring may have
> + * already been measured (for instance the same state existed earlier).
> + * But for SELinux the current data represents a state change and hence
> + * needs to be measured again. To enable this, pass a unique "event_name"
> + * to the IMA hook so that IMA subsystem will always measure the given data.
> + *
> + * For example,
> + * At time T0 SELinux data to be measured is "foo". IMA measures it.
> + * At time T1 the data is changed to "bar". IMA measures it.
> + * At time T2 the data is changed to "foo" again. IMA will not measure it
> + * (since it was already measured) unless the event_name, for instance,
> + * is different in this call.
> + */
> +static char *selinux_event_name(const char *name_prefix)
> +{
> + char *event_name = NULL;
> + struct timespec64 cur_time;
> +
> + ktime_get_real_ts64(&cur_time);
> + event_name = kasprintf(GFP_KERNEL, "%s-%lld:%09ld", name_prefix,
> + cur_time.tv_sec, cur_time.tv_nsec);
> + if (!event_name) {
> + pr_err("%s: event name not allocated.\n", __func__);
> + return NULL;
> + }
> +
> + return event_name;
> +}
> +
> +/*
> + * selinux_measure_state - Measure hash of the SELinux policy
> + *
> + * @state: selinux state struct
> + *
> + * NOTE: This function must be called with policy_mutex held.
> + */
> +void selinux_measure_state(struct selinux_state *state)
> +{
> + void *policy = NULL;
> + char *policy_event_name = NULL;
> + size_t policy_len;
> + int rc = 0;
> + bool initialized = selinux_initialized(state);
> +
> + /*
> + * Measure SELinux policy only after initialization is completed.
> + */
> + if (!initialized)
> + goto out;
> +
> + policy_event_name = selinux_event_name("selinux-policy-hash");
> + if (!policy_event_name) {
> + pr_err("%s: Event name for policy not allocated.\n",
> + __func__);
If the kasprintf() in selinux_event_name() fails, we'll get two similar
error messages saying that the event name could not be allocated. One of
these error messages can be removed.
> + rc = -ENOMEM;
> + goto out;
> + }
> +
> + rc = security_read_policy_kernel(state, &policy, &policy_len);
> + if (rc) {
> + pr_err("%s: Failed to read policy %d.\n", __func__, rc);
The calls to pr_err() in this aren't quite following the style of the
other error SELinux error messages.
$ git grep pr_err security/selinux
security/selinux/hooks.c: pr_err("SELinux: out of range capability %d\n", cap);
security/selinux/hooks.c: pr_err("SELinux: unable to map context to SID"
security/selinux/netlink.c: pr_err("SELinux: OOM in %s\n", __func__);
security/selinux/selinuxfs.c: pr_err("SELinux: Runtime disable is deprecated, use selinux=0 on the kernel cmdline.\n");
security/selinux/selinuxfs.c: pr_err("SELinux: failed to load policy booleans\n");
security/selinux/selinuxfs.c: pr_err("SELinux: failed to load policy classes\n");
...
security/selinux/ss/services.c: pr_err("SELinux: %s: unrecognized SID %d\n",
security/selinux/ss/services.c: pr_err("SELinux: %s: unrecognized SID %d\n",
security/selinux/ss/services.c: pr_err("SELinux: %s: unrecognized SID %d\n",
security/selinux/ss/services.c: pr_err("SELinux: %s: unrecognized SID %d\n",
security/selinux/ss/services.c: pr_err("SELinux: %s: unrecognized class %s\n",
Prepending your error message strings with "SELinux: " and lowercasing the
first character after "%s: " ought to do it.
All the other code changes in this patch look correct to me.
Tyler
> + 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/ss/services.c b/security/selinux/ss/services.c
> index 9704c8a32303..dfa2e00894ae 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -2180,6 +2180,7 @@ 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_measure_state(state);
> }
>
> void selinux_policy_commit(struct selinux_state *state,
> @@ -3875,8 +3876,33 @@ int security_netlbl_sid_to_secattr(struct selinux_state *state,
> }
> #endif /* CONFIG_NETLABEL */
>
> +/**
> + * security_read_selinux_policy - read the policy.
> + * @policy: SELinux policy
> + * @data: binary policy data
> + * @len: length of data in bytes
> + *
> + */
> +static int security_read_selinux_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 +3911,6 @@ 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 +3922,43 @@ int security_read_policy(struct selinux_state *state,
> if (!*data)
> return -ENOMEM;
>
> - fp.data = *data;
> - fp.len = *len;
> + return security_read_selinux_policy(policy, *data, len);
> +}
>
> - rc = policydb_write(&policy->policydb, &fp);
> - if (rc)
> - return rc;
> +/**
> + * security_read_policy_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_policy_kernel(struct selinux_state *state,
> + void **data, size_t *len)
> +{
> + struct selinux_policy *policy;
> + int rc = 0;
>
> - *len = (unsigned long)fp.data - (unsigned long)*data;
> - return 0;
> + policy = rcu_dereference_protected(
> + state->policy, lockdep_is_held(&state->policy_mutex));
> + if (!policy) {
> + rc = -EINVAL;
> + goto out;
> + }
> +
> + *len = policy->policydb.len;
> + *data = vmalloc(*len);
> + if (!*data) {
> + rc = -ENOMEM;
> + goto out;
> + }
>
> + rc = security_read_selinux_policy(policy, *data, len);
> +
> +out:
> + return rc;
> }
> --
> 2.17.1
>
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