Re: [PATCH v14 6/6] LSM: Multiple LSM Documentation and cleanup

[Casey Schaufler <casey@schaufler-ca.com>]

On 7/26/2013 4:17 PM, Randy Dunlap wrote:
> On 07/25/13 11:32, Casey Schaufler wrote:
>> Subject: [PATCH v14 6/6] LSM: Multiple LSM Documentation and cleanup
>>
>> Add documentation and remove the obsolete capability LSM.
>> Clean up some comments in security.h
>>
>> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
>>
>> ---
>>  Documentation/security/LSM.txt |   56 +-
>>  include/linux/security.h       |   48 +-
>>  security/Makefile              |    1 -
>>  security/capability.c          | 1106 ----------------------------------------
>>  4 files changed, 77 insertions(+), 1134 deletions(-)
>>
>> diff --git a/Documentation/security/LSM.txt b/Documentation/security/LSM.txt
>> index c335a76..69cf466 100644
>> --- a/Documentation/security/LSM.txt
>> +++ b/Documentation/security/LSM.txt
>> @@ -7,20 +7,56 @@ various security checks to be hooked by new kernel extensions. The name
>>  loadable kernel modules. Instead, they are selectable at build-time via
>>  CONFIG_DEFAULT_SECURITY and can be overridden at boot-time via the
>>  "security=..." kernel command line argument, in the case where multiple
>> -LSMs were built into a given kernel.
>> +LSMs were built into a given kernel. The names of the active LSMs
>> +can be read from /sys/kernel/security/lsm.
>> +
>> +Both CONFIG_DEFAULT_SECURITY and the "security=" option take a comma
>> +separated list of LSM names. The LSM hooks are invoked in the order
>> +specified. All hooks provided are invoked regardless of the outcome
>> +of preceding hooks. Hooks that return success or failure results
>> +return success if all of the LSM provided hooks succeed and the error
>> +code of the last failing hook on error.
>> +
>> +Information from an LSM can come in one of two forms. The raw data
>> +used by the LSM is typically the preferred form. SELinux contexts and
>> +Smack labels are examples of raw LSM data. If the data from multiple
>> +LSMs is presented together it will be in the form:
>> +
>> +	lsmname='value'[lsmname='value']...
> no commas? just (made up example):
>
> 	smack='label'selinux='notstrict'

That's correct. No commas. Because all values are quoted commas
are unnecessary. If I want the SELinux portion of a context I can call

	sscanf(source, "selinux='%[^']'", result);

and comma separation helps not a bit.

>> +
>> +Interfaces that accept LSM data as input accept this format as well,
>> +passing only the relevant portion of the data to each LSM.
>> +
>> +The /proc filesystem attribute interface supports files from a time
>> +when only one LSM could be used at a time. CONFIG_PRESENT_SECURITY
>> +defines which LSM uses these interfaces. The name of this LSM can be
>> +read from /sys/kernel/security/present. There are also LSM identified
>> +interfaces which should be used in preference to the undifferentiated
>> +interfaces. The attribute interface "context" always provides the
>> +data from all LSMs that maintain it in the lsmname='value' format.
>> +
>> +The three networking mechanisms supporting LSM attributes are
>> +restricted to providing those attributes for a single LSM each.
>> +CONFIG_SECMARK_LSM specifies which LSM will provide hooks for the
>> +secmark mechanism. CONFIG_NETLABEL_LSM specifies which LSM hooks
>> +are used by NetLabel to provide IPv4 CIPSO headers. CONFIG_XFRM_LSM
>> +specifies the LSM providing xfrm hooks. CONFIG_PEERSEC_LSM allows
>> +for either a specific LSM to provide data with SO_PEERSEC or for
>> +all LSMs that provide data to do so.
>> +
>> +The Linux capabilities system is used in conjunction with any LSMs.
>> +LSM hooks are called after the capability checks in most cases,
>                         ^^^^^
>> +but after in a small number of cases. All LSM hooks need to be aware
>        ^^^^^
>
>        one of these 'after's should be 'before' ??

Yup. The latter should in fact be "before".

>> +of the potential interactions with the capability system. For more
>> +details on capabilities, see capabilities(7) in the Linux man-pages
>> +project.
>>  
>>  The primary users of the LSM interface are Mandatory Access Control
>>  (MAC) extensions which provide a comprehensive security policy. Examples
>>  include SELinux, Smack, Tomoyo, and AppArmor. In addition to the larger
>> -MAC extensions, other extensions can be built using the LSM to provide
>> -specific changes to system operation when these tweaks are not available
>> -in the core functionality of Linux itself.
>> -
>> -Without a specific LSM built into the kernel, the default LSM will be the
>> -Linux capabilities system. Most LSMs choose to extend the capabilities
>> -system, building their checks on top of the defined capability hooks.
>> -For more details on capabilities, see capabilities(7) in the Linux
>> -man-pages project.
>> +MAC extensions, other extensions such as Yama can be built using the LSM
>> +to provide specific changes to system operation when these tweaks are not
>> +available in the core functionality of Linux itself.
>>  
>>  Based on http://kerneltrap.org/Linux/Documenting_Security_Module_Intent,
>>  a new LSM is accepted into the kernel when its intent (a description of
>>


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