Re: Linux Firmware Signing

[Luis R. Rodriguez]

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:

SECURITY_FILE_SYSDATA_FW
SECURITY_FILE_SYSDATA_MICROCODE
SECURITY_FILE_SYSDATA_EEPROM
SECURITY_FILE_SYSDATA_POLICY (for 802.11 regulatory I suppose)

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
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