Re: Linux Firmware Signing

[Luis R. Rodriguez]

On Wed, Sep 02, 2015 at 08:05:36PM -0400, Mimi Zohar wrote:
> On Wed, 2015-09-02 at 20:46 +0200, Luis R. Rodriguez 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.
> 
> The names are based on the calling security hook.   For a description of
> each of these security hooks refer to include/linux/lsm_hooks.h.

I see, thanks, ok so BPRM_CHECK = for binary loading, are you folks
really wanting to unify LSM hooks for firmware, modules, and binary
data ?

POST_SETATTR seems to be for inode_post_setxattr, so that as well?

POLICY_CHECK seems broad, not sure what to relate that to exactly.
Is this just SELinux polify files? Or is this something more broad?

> > 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
> >  * @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.
> 
> Oh, I'm really confused as to why MOK would be a separate hook.  I
> thought the discussion was about using a key in the UEFI MOK DB for
> verifying locally signed files.

That's correct, and no I was not thinking of a separate hook but rather
a type that lets the LSM know that MOK was used to sign the file consumed.

  Luis
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