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Message-ID: <c0fbfcf3-ec36-872a-c389-b3fea214848c@linux.microsoft.com>
Date: Mon, 20 Jul 2020 08:17:28 -0700
From: Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
To: Stephen Smalley <stephen.smalley.work@...il.com>
Cc: Mimi Zohar <zohar@...ux.ibm.com>,
Casey Schaufler <casey@...aufler-ca.com>,
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 v3 4/5] LSM: Define SELinux function to measure security
state
On 7/20/20 7:31 AM, Stephen Smalley wrote:
>> +void __init selinux_init_measurement(void)
>> +{
>> + int i;
>> +
>> + /*
>> + * enabled
>> + * enforcing
>> + * checkreqport
>
> checkreqprot (spelling)
:( - will fix that.
>
> What about initialized? Or do you consider that to be implicitly
> true/1 else we wouldn't be taking a measurement? Only caveat there is
> that it provides one more means of disabling measurements (at the same
> time as disabling enforcement) by setting it to false/0 via kernel
> write flaw.
Yes - I was thinking measuring SELinux state would be meaningful only
when initialized is set to true/1.
I can include "initialized" as well in the measurement.
>
>> + * All policy capability flags
>> + */
>> + selinux_state_count = 3 + __POLICYDB_CAPABILITY_MAX;
>> +
>> + selinux_state_string_len = snprintf(NULL, 0, str_format,
>> + "enabled", 0);
>> + selinux_state_string_len += snprintf(NULL, 0, str_format,
>> + "enforcing", 0);
>> + selinux_state_string_len += snprintf(NULL, 0, str_format,
>> + "checkreqprot", 0);
>> + for (i = 3; i < selinux_state_count; i++) {
>> + selinux_state_string_len +=
>> + snprintf(NULL, 0, str_format,
>> + selinux_policycap_names[i-3], 0);
>> + }
>
> What's the benefit of this pattern versus just making the loop go from
> 0 to __POLICYDB_CAPABILITY_MAX and using selinux_policycap_names[i]?
No real benefit - I was just trying to use selinux_state_count.
I'll change the loop to go from 0 to POLICY_CAP_MAX
>
>> +void selinux_measure_state(struct selinux_state *selinux_state)
>> +{
>> + void *policy = NULL;
>> + void *policy_hash = NULL;
>> + size_t curr, buflen;
>> + int i, policy_hash_len, rc = 0;
>> +
>> + if (!selinux_initialized(selinux_state)) {
>> + pr_warn("%s: SELinux not yet initialized.\n", __func__);
>> + return;
>> + }
>
> We could measure the global state variables before full SELinux
> initialization (i.e. policy load).
> Only the policy hash depends on having loaded the policy.
Thanks for the information. I'll measure the state variables always and
measure policy only if "initialized" is true/1.
>
>> +
>> + if (!selinux_state_string) {
>> + pr_warn("%s: Buffer for state not allocated.\n", __func__);
>> + return;
>> + }
>> +
>> + curr = snprintf(selinux_state_string, selinux_state_string_len,
>> + str_format, "enabled",
>> + !selinux_disabled(selinux_state));
>> + curr += snprintf((selinux_state_string + curr),
>> + (selinux_state_string_len - curr),
>> + str_format, "enforcing",
>> + enforcing_enabled(selinux_state));
>> + curr += snprintf((selinux_state_string + curr),
>> + (selinux_state_string_len - curr),
>> + str_format, "checkreqprot",
>> + selinux_checkreqprot(selinux_state));
>> +
>> + for (i = 3; i < selinux_state_count; i++) {
>> + curr += snprintf((selinux_state_string + curr),
>> + (selinux_state_string_len - curr),
>> + str_format,
>> + selinux_policycap_names[i - 3],
>> + selinux_state->policycap[i - 3]);
>> + }
>
> Same question here as for the previous loop; seems cleaner to go from
> 0 to __POLICYDB_CAPABILITY_MAX and use [i].
Will change it.
>
> What public git tree / branch would you recommend trying to use your
> patches against? Didn't seem to apply to any of the obvious ones.
>
Please try it on Mimi's next-integrity branch
https://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity.git/log/?h=next-integrity
You can try it on Linus's mainline as well if you apply the following
patch first (have mentioned that in the Cover letter as well)
https://patchwork.kernel.org/patch/11612989/
Thanks for trying out the changes. Please let me know the defects you find.
Just to let you know - I am making the following change (will update in
the next patch):
=> Save the last policy hash and state string in selinux_state struct.
=> Measure policy and hash only if it has changed since the last
measurement.
=> Also, suffix the IMA event name used with time stamp. For example,
10 e32e...5ac3 ima-buf sha256:86e8...4594
selinux-state-1595257807:874963248
656e61626c65643d313b656e666f7263696e673d303b636865636b72657170726f743d313b6e6574706565723d313b6f70656e7065726d3d313b657874736f636b636c6173733d313b616c776179736e6574776f726b3d303b6367726f75707365636c6162656c3d313b6e6e706e6f737569647472616e736974696f6e3d313b67656e66737365636c6162656c73796d6c696e6b3d303b
10 f4a7...9408 ima-buf sha256:4941...68fc
selinux-policy-hash-1595257807:874963248
8d1d...1834
The above will ensure the following sequence will be measured:
#1 State A - Measured
#2 Change from State A to State B - Measured
#3 Change from State B back to State A - Since the measured data is
same as in #1, the change will be measured only if the event name is
different between #1 and #3
thanks,
-lakshmi
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