Re: Linux Firmware Signing

[Kees Cook]

[removed bounced email addresses]

On Wed, Sep 2, 2015 at 2:37 PM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
> On Wed, Sep 02, 2015 at 01:54:43PM -0700, Kees Cook wrote:
>> On Wed, Sep 2, 2015 at 11:46 AM, Luis R. Rodriguez <mcgrof@xxxxxxxx> wrote:
>> > On Tue, Sep 01, 2015 at 11:35:05PM -0400, Mimi Zohar wrote:
>> >> > OK great, I think that instead of passing the actual routine name we should
>> >> > instead pass an enum type for to the LSM, that'd be easier to parse and we'd
>> >> > then have each case well documented. Each LSM then could add its own
>> >> > documetnation for this and can switch on it. If we went with a name we'd have
>> >> > to to use something like __func__ and then parse that, its not clear if we need
>> >> > to get that specific.
>> >>
>> >> Agreed.  IMA already defines an enumeration.
>> >>
>> >> /* IMA policy related functions */
>> >> enum ima_hooks { FILE_CHECK = 1, MMAP_CHECK, BPRM_CHECK, MODULE_CHECK,
>> >>                  FIRMWARE_CHECK, POLICY_CHECK, POST_SETATTR };
>> >>
>> >
>> > We want something that is not only useful for IMA but any other LSM,
>> > and FILE_CHECK seems very broad, not sure what BPRM_CHECK is even upon
>> > inspecting kernel code. Likewise for POST_SETATTR. POLICY_CHECK might
>> > be broad, perhaps its best we define then a generic set of enums to
>> > which IMA can map them to then and let it decide. This would ensure
>> > that the kernel defines each use caes for file inspection carefully,
>> > documents and defines them and if an LSM wants to bunch a set together
>> > it can do so easily with a switch statement to map set of generic
>> > file checks in kernel to a group it already handles.
>> >
>> > For instance at least in the short term we'd try to unify:
>> >
>> > security_kernel_fw_from_file()
>> > security_kernel_module_from_file()
>> >
>> > to perhaps:
>> >
>> > security_kernel_from_file()
>> >
>> > As far, as far as I can tell, the only ones we'd be ready to start
>> > grouping immediately or with small amount of work rather soon:
>> >
>> > /**
>> >  *
>> >  * enum security_filecheck - known kernel security file checks types
>> >  *
>> >  * @__SECURITY_FILECHECK_UNSPEC: attribute 0 reserved
>> >  * @SECURITY_FILECHECK_MODULE: the file being processed is a Linux kernel module
>> >  * @SECURITY_FILECHECK_SYSDATA: the file being processed is either a firmware
>> >  *      file or a system data file read from /lib/firmware/* by firmware_class
>>
>> I'd prefer a distinct category for firmware, as it carries an
>> implication that it is an executable blob of some sort (I know not all
>> are, though).
>
> The ship has sailed in terms of folks using frimrware API for things
> that are not-firmware per se. The first one I am aware of was the
> EEPROM override for the p54 driver. The other similar one was CPU
> microcode, but that's a bit more close to home with "firmware". We
> could ask users on the new system data request API I am building
> to describe the type of file being used, as I agree differentiating
> this for security purposes might be important. So other than just
> file type we could have sub type category, then we could have,
>
> SECURITY_FILECHECK_SYSDATA, and then:

I object to executable code being called data. :)

> SECURITY_FILE_SYSDATA_FW
> SECURITY_FILE_SYSDATA_MICROCODE
> SECURITY_FILE_SYSDATA_EEPROM
> SECURITY_FILE_SYSDATA_POLICY (for 802.11 regulatory I suppose)

The exception to the firmware loading is data, so the primary name
should be firmware. Regardless, if we want distinct objects, just name
them:

SECURITY_FILE_FIRMWARE
SECURITY_FILE_SYSDATA

Do we need finer-grain sub types?

>
> If we do this then we could juse have:
>
> SECURITY_FILECHECK_KEXEC and on that have substypes:
>
> SECURITY_FILE_KEXEC_KERNEL
> SECURITY_FILE_KEXEC_INITRAMFS
>
> Would that be desirable and help grow this to be easily extensible?
>
>   Luis

-Kees

-- 
Kees Cook
Chrome OS Security
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