linux-kernel - Re: [PATCH] SELinux: Measure state and hash of policy using IMA

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Message-ID: <7c4e2e9f-54e1-1dee-c33c-64dac0fe9678@linux.microsoft.com>
Date:   Mon, 7 Sep 2020 21:44:37 -0700
From:   Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
To:     Stephen Smalley <stephen.smalley.work@...il.com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>, Paul Moore <paul@...l-moore.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        tusharsu@...ux.microsoft.com, Sasha Levin <sashal@...nel.org>,
        James Morris <jmorris@...ei.org>,
        linux-integrity@...r.kernel.org,
        SElinux list <selinux@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] SELinux: Measure state and hash of policy using IMA

On 9/7/20 3:32 PM, Stephen Smalley wrote:

>> Signed-off-by: Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
>> Suggested-by: Stephen Smalley <stephen.smalley.work@...il.com>
>> Reported-by: kernel test robot <lkp@...el.com> # error: implicit declaration of function 'vfree'
>> Reported-by: kernel test robot <lkp@...el.com> # error: implicit declaration of function 'crypto_alloc_shash'
>> Reported-by: kernel test robot <lkp@...el.com> # sparse: symbol 'security_read_selinux_policy' was not declared. Should it be static?
> 
> Not sure these Reported-by lines are useful since they were just on
> submitted versions of the patch not on an actual merged commit.

I'll remove them when I update the patch.

> 
>> diff --git a/security/selinux/measure.c b/security/selinux/measure.c
>> new file mode 100644
>> index 000000000000..caf9107937d9
>> --- /dev/null
>> +++ b/security/selinux/measure.c
> <snip>
>> +void selinux_measure_state(struct selinux_state *state, bool policy_mutex_held)
>> +{
> <snip>
>> +
>> +       if (!policy_mutex_held)
>> +               mutex_lock(&state->policy_mutex);
>> +
>> +       rc = security_read_policy_kernel(state, &policy, &policy_len);
>> +
>> +       if (!policy_mutex_held)
>> +               mutex_unlock(&state->policy_mutex);
> 
> This kind of conditional taking of a mutex is generally frowned upon
> in my experience.
> You should likely just always take the mutex in the callers of
> selinux_measure_state() instead.
> In some cases, it may be the caller of the caller.  Arguably selinuxfs
> could be taking it around all state modifying operations (e.g.
> enforce, checkreqprot) not just policy modifying ones although it
> isn't strictly for that purpose.

Since currently policy_mutex is not used to synchronize access to state 
variables (enforce, checkreqprot, etc.) I am wondering if 
selinux_measure_state() should measure only state if policy_mutex is not 
held by the caller - similar to how we skip measuring policy if 
initialization is not yet completed.

	/*
	 * Measure SELinux policy only after initialization is
          * completed.
	 */
	if (!initialized)
		goto out;

  -lakshmi