diff mbox series

[v9,8/8] selinux: include a consumer of the new IMA critical data hook

Message ID 20201212180251.9943-9-tusharsu@linux.microsoft.com (mailing list archive)
State New
Headers show
Series IMA: support for measuring kernel integrity critical data | expand

Commit Message

Tushar Sugandhi Dec. 12, 2020, 6:02 p.m. UTC
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.

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@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/include/security.h  | 11 +++-
 security/selinux/measure.c           | 79 ++++++++++++++++++++++++++++
 security/selinux/ss/services.c       | 71 +++++++++++++++++++++----
 5 files changed, 155 insertions(+), 11 deletions(-)
 create mode 100644 security/selinux/measure.c

Comments

Paul Moore Dec. 23, 2020, 9:10 p.m. UTC | #1
On Sat, Dec 12, 2020 at 1:03 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.
>
> 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@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/include/security.h  | 11 +++-
>  security/selinux/measure.c           | 79 ++++++++++++++++++++++++++++
>  security/selinux/ss/services.c       | 71 +++++++++++++++++++++----
>  5 files changed, 155 insertions(+), 11 deletions(-)
>  create mode 100644 security/selinux/measure.c

...

> 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

Naming things is hard, I get that, but I would prefer if we just
called this file "ima.c" or something similar.  The name "measure.c"
implies a level of abstraction or general use which simply doesn't
exist here.  Let's help make it a bit more obvious what should belong
in this file.

> 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

If you are going to put the SELinux/IMA function(s) into a separate
source file, please put the function declarations into a separate
header file too.  For example, look at
security/selinux/include/{netif,netnode,netport,etc.}.h.

> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
> new file mode 100644
> index 000000000000..b7e24358e11d
> --- /dev/null
> +++ b/security/selinux/measure.c
> @@ -0,0 +1,79 @@
> +// 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)
> +{
> +       struct timespec64 cur_time;
> +
> +       ktime_get_real_ts64(&cur_time);
> +       return kasprintf(GFP_KERNEL, "%s-%lld:%09ld", name_prefix,
> +                        cur_time.tv_sec, cur_time.tv_nsec);
> +}

Why is this a separate function?  It's three lines long and only
called from selinux_measure_state().  Do you ever see the SELinux/IMA
code in this file expanding to the point where this function is nice
from a reuse standpoint?

Also, I assume you are not concerned about someone circumventing the
IMA measurements by manipulating the time?  In most systems I would
expect the time to be a protected entity, but with many systems
getting their time from remote systems I thought it was worth
mentioning.

> +/*
> + * 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)

Similar to the name of this source file, let's make it clear this is
for IMA.  How about calling this selinux_ima_measure_state() or
similar?

> +{
> +       void *policy = NULL;
> +       char *policy_event_name = NULL;
> +       size_t policy_len;
> +       int rc = 0;
> +       bool initialized = selinux_initialized(state);

Why bother with the initialized variable?  Unless I'm missing
something it is only used once in the code below.

> +       /*
> +        * 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("SELinux: %s: event name for policy not allocated.\n",
> +                      __func__);
> +               rc = -ENOMEM;

This function doesn't return an error code, why bother with setting
the rc variable here?

> +               goto out;
> +       }
> +
> +       rc = security_read_policy_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/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)

Let's just call this "security_read_policy()".

> +{
> +       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)> --
> 2.17.1
>

>                 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)

Let's call this "security_read_state_kernel()".

> +{
> +       struct selinux_policy *policy;
> +       int rc = 0;

See below, the rc variable is not needed.

> -       *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;

Jumping to the out label is a little silly since it is just a return;
do a "return -EINVAL;" here instead.

> +       }
> +
> +       *len = policy->policydb.len;
> +       *data = vmalloc(*len);
> +       if (!*data) {
> +               rc = -ENOMEM;
> +               goto out;

Same as above, "return -ENOMEM;" please.

> +       }
>
> +       rc = security_read_selinux_policy(policy, *data, len);

You should be able to do "return security_read_selinux_policy(...);" here.

> +
> +out:
> +       return rc;
>  }
Lakshmi Ramasubramanian Jan. 4, 2021, 11:30 p.m. UTC | #2
On 12/23/20 1:10 PM, Paul Moore wrote:

Hi Paul,

> ...
> 
>> 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
> 
> Naming things is hard, I get that, but I would prefer if we just
> called this file "ima.c" or something similar.  The name "measure.c"
> implies a level of abstraction or general use which simply doesn't
> exist here.  Let's help make it a bit more obvious what should belong
> in this file.
Agreed - I will rename the file to ima.c

> 
>> 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
> 
> If you are going to put the SELinux/IMA function(s) into a separate
> source file, please put the function declarations into a separate
> header file too.  For example, look at
> security/selinux/include/{netif,netnode,netport,etc.}.h.

I will create a new header file "security/selinux/include/ima.h" and 
move the function declarations for IMA functions to that header.

> 
>> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
>> new file mode 100644
>> index 000000000000..b7e24358e11d
>> --- /dev/null
>> +++ b/security/selinux/measure.c
>> @@ -0,0 +1,79 @@
>> +// 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)
>> +{
>> +       struct timespec64 cur_time;
>> +
>> +       ktime_get_real_ts64(&cur_time);
>> +       return kasprintf(GFP_KERNEL, "%s-%lld:%09ld", name_prefix,
>> +                        cur_time.tv_sec, cur_time.tv_nsec);
>> +}
> 
> Why is this a separate function?  It's three lines long and only
> called from selinux_measure_state().  Do you ever see the SELinux/IMA
> code in this file expanding to the point where this function is nice
> from a reuse standpoint?

Earlier I had two measurements - one for SELinux configuration/state and 
another for SELinux policy. selinux_event_name() was used to generate 
event name for each of them.

In this patch set I have included only one measurement - for SELinux 
policy. I plan to add "SELinux configuration/state" measurement in a 
separate patch - I can reuse selinux_event_name() in that patch.

Also, I think the comment in the function header for 
selinux_event_name() is useful.

I prefer to have a separate function, if that's fine by you.

> 
> Also, I assume you are not concerned about someone circumventing the
> IMA measurements by manipulating the time?  In most systems I would
> expect the time to be a protected entity, but with many systems
> getting their time from remote systems I thought it was worth
> mentioning.
I am using time function to generate a unique name for the IMA 
measurement event, such as, "selinux-policy-hash-1609790281:860232824". 
This is to ensure that state changes in SELinux data are always measured.

If you think time manipulation can be an issue, please let me know a 
better way to generate unique event names.

> 
>> +/*
>> + * 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)
> 
> Similar to the name of this source file, let's make it clear this is
> for IMA.  How about calling this selinux_ima_measure_state() or
> similar?
Sure - I will change the function name to selinux_ima_measure_state().

> 
>> +{
>> +       void *policy = NULL;
>> +       char *policy_event_name = NULL;
>> +       size_t policy_len;
>> +       int rc = 0;
>> +       bool initialized = selinux_initialized(state);
> 
> Why bother with the initialized variable?  Unless I'm missing
> something it is only used once in the code below.
You are right - I will remove "initialized" variable and directly get 
the state using selinux_initialized().

> 
>> +       /*
>> +        * 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("SELinux: %s: event name for policy not allocated.\n",
>> +                      __func__);
>> +               rc = -ENOMEM;
> 
> This function doesn't return an error code, why bother with setting
> the rc variable here?
Yes - it is not necessary. I will remove the line.

> 
>> +               goto out;
>> +       }
>> +
>> +       rc = security_read_policy_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/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)
> 
> Let's just call this "security_read_policy()".
There is another function in this file with the name security_read_policy().

How about changing the above function name to "read_selinux_policy()" 
since this is a local/static function.

> 
>> +{
>> +       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)> --
>> 2.17.1
>>
> 
>>                  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)
> 
> Let's call this "security_read_state_kernel()".
Sure - I will rename the function.

> 
>> +{
>> +       struct selinux_policy *policy;
>> +       int rc = 0;
> 
> See below, the rc variable is not needed.
> 
>> -       *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;
> 
> Jumping to the out label is a little silly since it is just a return;
> do a "return -EINVAL;" here instead.
> 
>> +       }
>> +
>> +       *len = policy->policydb.len;
>> +       *data = vmalloc(*len);
>> +       if (!*data) {
>> +               rc = -ENOMEM;
>> +               goto out;
> 
> Same as above, "return -ENOMEM;" please.
> 
>> +       }
>>
>> +       rc = security_read_selinux_policy(policy, *data, len);
> 
> You should be able to do "return security_read_selinux_policy(...);" here.

I will remove the local variable "rc" and make the three changes you've 
stated above.

Thanks for reviewing the changes.

  -lakshmi

> 
>> +
>> +out:
>> +       return rc;
>>   }
>
Paul Moore Jan. 5, 2021, 2:13 a.m. UTC | #3
On Mon, Jan 4, 2021 at 6:30 PM Lakshmi Ramasubramanian
<nramas@linux.microsoft.com> wrote:
> On 12/23/20 1:10 PM, Paul Moore wrote:
> Hi Paul,

Hello.

> >> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
> >> new file mode 100644
> >> index 000000000000..b7e24358e11d
> >> --- /dev/null
> >> +++ b/security/selinux/measure.c
> >> @@ -0,0 +1,79 @@
> >> +// 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)
> >> +{
> >> +       struct timespec64 cur_time;
> >> +
> >> +       ktime_get_real_ts64(&cur_time);
> >> +       return kasprintf(GFP_KERNEL, "%s-%lld:%09ld", name_prefix,
> >> +                        cur_time.tv_sec, cur_time.tv_nsec);
> >> +}
> >
> > Why is this a separate function?  It's three lines long and only
> > called from selinux_measure_state().  Do you ever see the SELinux/IMA
> > code in this file expanding to the point where this function is nice
> > from a reuse standpoint?
>
> Earlier I had two measurements - one for SELinux configuration/state and
> another for SELinux policy. selinux_event_name() was used to generate
> event name for each of them.
>
> In this patch set I have included only one measurement - for SELinux
> policy. I plan to add "SELinux configuration/state" measurement in a
> separate patch - I can reuse selinux_event_name() in that patch.

I'm curious about this second measurement.  My apologies if you posted
it previously, this patchset has gone through several iterations and
simply can't recall all the different versions without digging through
the list archives.

Is there a reason why the second measurement isn't included in this
patch?  Or this patchset if it is too big to be a single patch?

> Also, I think the comment in the function header for
> selinux_event_name() is useful.
>
> I prefer to have a separate function, if that's fine by you.

Given just this patch I would prefer if you folded
selinux_event_name() into selinux_measure_state().  However, I agree
with you that the comments in the selinux_event_name() header block is
useful, I would suggest moving those into the body of
selinux_measure_state() directly above the calls to
ktime_get_real_ts64() and kasprintf().

> > Also, I assume you are not concerned about someone circumventing the
> > IMA measurements by manipulating the time?  In most systems I would
> > expect the time to be a protected entity, but with many systems
> > getting their time from remote systems I thought it was worth
> > mentioning.
>
> I am using time function to generate a unique name for the IMA
> measurement event, such as, "selinux-policy-hash-1609790281:860232824".
> This is to ensure that state changes in SELinux data are always measured.
>
> If you think time manipulation can be an issue, please let me know a
> better way to generate unique event names.

Yes, I understand that you are using the time value as a way of
ensuring you always have a different event name and hence a new
measurement.  However, I was wondering if you would be okay if the
time was adjusted such that an event name was duplicated and a
measurement missed?  Is that a problem for you?  It seems like it
might be an issue, but you and Mimi know IMA better than I do.

> >> 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
> >> @@ -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)
> >
> > Let's just call this "security_read_policy()".
> There is another function in this file with the name security_read_policy().
>
> How about changing the above function name to "read_selinux_policy()"
> since this is a local/static function.

Ooops, sorry about that!  I'm not sure what I was thinking there :)

How about "__security_read_policy()"?
Lakshmi Ramasubramanian Jan. 5, 2021, 5:24 a.m. UTC | #4
On 1/4/21 6:13 PM, Paul Moore wrote:
> On Mon, Jan 4, 2021 at 6:30 PM Lakshmi Ramasubramanian
> <nramas@linux.microsoft.com> wrote:
>> On 12/23/20 1:10 PM, Paul Moore wrote:
>> Hi Paul,
> 
> Hello.
> 
>>>> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
>>>> new file mode 100644
>>>> index 000000000000..b7e24358e11d
>>>> --- /dev/null
>>>> +++ b/security/selinux/measure.c
>>>> @@ -0,0 +1,79 @@
>>>> +// 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)
>>>> +{
>>>> +       struct timespec64 cur_time;
>>>> +
>>>> +       ktime_get_real_ts64(&cur_time);
>>>> +       return kasprintf(GFP_KERNEL, "%s-%lld:%09ld", name_prefix,
>>>> +                        cur_time.tv_sec, cur_time.tv_nsec);
>>>> +}
>>>
>>> Why is this a separate function?  It's three lines long and only
>>> called from selinux_measure_state().  Do you ever see the SELinux/IMA
>>> code in this file expanding to the point where this function is nice
>>> from a reuse standpoint?
>>
>> Earlier I had two measurements - one for SELinux configuration/state and
>> another for SELinux policy. selinux_event_name() was used to generate
>> event name for each of them.
>>
>> In this patch set I have included only one measurement - for SELinux
>> policy. I plan to add "SELinux configuration/state" measurement in a
>> separate patch - I can reuse selinux_event_name() in that patch.
> 
> I'm curious about this second measurement.  My apologies if you posted
> it previously, this patchset has gone through several iterations and
> simply can't recall all the different versions without digging through
> the list archives.
> 

The 2nd measurement is for SELinux state data such as "enforcing", 
"checkreqprot", policycap[__POLICYDB_CAPABILITY_MAX], etc.

> Is there a reason why the second measurement isn't included in this
> patch?  Or this patchset if it is too big to be a single patch?
> 

For illustrating the use of the new IMA hook, that my colleague Tushar 
has added for measuring kernel critical data, we have included only one 
SELinux measurement in this patch set - the measurement of SELinux 
policy. This also helped in keeping this patch smaller.

When this patch set is merged, I'll post a separate patch to add 
measurement of SELinux state data I have mentioned above.

>> Also, I think the comment in the function header for
>> selinux_event_name() is useful.
>>
>> I prefer to have a separate function, if that's fine by you.
> 
> Given just this patch I would prefer if you folded
> selinux_event_name() into selinux_measure_state().  However, I agree
> with you that the comments in the selinux_event_name() header block is
> useful, I would suggest moving those into the body of
> selinux_measure_state() directly above the calls to
> ktime_get_real_ts64() and kasprintf().

Sure - I will make that change.

> 
>>> Also, I assume you are not concerned about someone circumventing the
>>> IMA measurements by manipulating the time?  In most systems I would
>>> expect the time to be a protected entity, but with many systems
>>> getting their time from remote systems I thought it was worth
>>> mentioning.
>>
>> I am using time function to generate a unique name for the IMA
>> measurement event, such as, "selinux-policy-hash-1609790281:860232824".
>> This is to ensure that state changes in SELinux data are always measured.
>>
>> If you think time manipulation can be an issue, please let me know a
>> better way to generate unique event names.
> 
> Yes, I understand that you are using the time value as a way of
> ensuring you always have a different event name and hence a new
> measurement.  However, I was wondering if you would be okay if the
> time was adjusted such that an event name was duplicated and a
> measurement missed?  Is that a problem for you?  It seems like it
> might be an issue, but you and Mimi know IMA better than I do.

If the system time was adjusted such that the event name is duplicated, 
we could miss measurements - this is not okay.

For example:
  #1 Say, at time T1 SELinux state being measured is "foo" - IMA will 
measure it.
  #2 at time T2, the state changes to "bar" - IMA will measure it
  #3 at time T3, the state changes from "bar" to "foo" again. Unless the 
"event name" passed in the measurement call is different from what was 
passed in step #1, IMA will not measure it and hence we'll miss the 
state change.

If system time can be manipulated to return the same "timer tick" on 
every call to ktime_get_real_ts64(), we will lose measurement in Step #3 
above.

But given that we are using ktime_get_real_ts64() to get the timer tick, 
is it possible to manipulate the system time without compromising the 
overall functioning of the rest of the system? If yes, then it is an 
issue - I mean, there is a possibility of losing some measurements.

> 
>>>> 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
>>>> @@ -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)
>>>
>>> Let's just call this "security_read_policy()".
>> There is another function in this file with the name security_read_policy().
>>
>> How about changing the above function name to "read_selinux_policy()"
>> since this is a local/static function.
> 
> Ooops, sorry about that!  I'm not sure what I was thinking there :)
> 
> How about "__security_read_policy()"?
> 

Sure - I will change the function name to "__security_read_policy()".

thanks,
  -lakshmi
diff mbox series

Patch

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..b7e24358e11d
--- /dev/null
+++ b/security/selinux/measure.c
@@ -0,0 +1,79 @@ 
+// 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)
+{
+	struct timespec64 cur_time;
+
+	ktime_get_real_ts64(&cur_time);
+	return kasprintf(GFP_KERNEL, "%s-%lld:%09ld", name_prefix,
+			 cur_time.tv_sec, cur_time.tv_nsec);
+}
+
+/*
+ * 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("SELinux: %s: event name for policy not allocated.\n",
+		       __func__);
+		rc = -ENOMEM;
+		goto out;
+	}
+
+	rc = security_read_policy_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/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;
 }