Re: Linux Firmware Signing

From: Kees Cook
Date: Wed Sep 02 2015 - 16:54:51 EST

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 */
> 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).


> * @SECURITY_FILECHECK_KEXEC_KERNEL: the file being processed is a kernel file
> * used by kexec
> * @SECURITY_FILECHECK_KEXEC_INITRAMFS: the file being processed is an initramfs
> * used by kexec
> * The kernel reads files directly from the filesystem for a series of
> * operations. The list of files the kernel reads from the filesystem are
> * limited and each type of file consumed may have a different format and
> * security vetting procedures. The kernel enables LSMs to vet for these files
> * through a shared LSM hook prior to consumption. This list documents the
> * different special kernel file types read by the kernel, it enables LSMs
> * to vet for each differently if needed.
> enum security_filecheck {
> };
> Provided the MOK thing or alternative gets addressed we could also soon add
> something for SELinux policy files but that needs to be discussed further
> it seems. If MOK is used would SECURITY_FILECHECK_POLICY_MOK be OK? Again
> this would likely need further discussion, its why I didn't list it above.
> Luis

Kees Cook
Chrome OS Security
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