Re: Linux Firmware Signing

[Kees Cook]

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).

-Kees

>  * @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 {
>         SECURITY_FILECHECK_UNSPEC,
>         SECURITY_FILECHECK_MODULE,
>         SECURITY_FILECHECK_SYSDATA,
>         SECURITY_FILECHECK_KEXEC_KERNEL,
>         SECURITY_FILECHECK_KEXEC_INITRAMFS,
> };
>
> 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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