Re: [PATCH v3 12/19] firmware_loader: Use security_post_load_data()

Kees Cook
2020-07-28T19:44:01Z

On Mon, Jul 27, 2020 at 06:57:45AM -0400, Mimi Zohar wrote:
> On Fri, 2020-07-24 at 14:36 -0700, Kees Cook wrote:
> > Now that security_post_load_data() is wired up, use it instead
> > of the NULL file argument style of security_post_read_file(),
> > and update the security_kernel_load_data() call to indicate that a
> > security_kernel_post_load_data() call is expected.
> >
> > Wire up the IMA check to match earlier logic. Perhaps a generalized
> > change to ima_post_load_data() might look something like this:
> >
> > return process_buffer_measurement(buf, size,
> > kernel_load_data_id_str(load_id),
> > read_idmap[load_id] ?: FILE_CHECK,
> > 0, NULL);
> >
> > Signed-off-by: Kees Cook <keescook@chromium.org>
>
> process_measurement() measures, verifies a file signature -  both
> signatures stored as an xattr and as an appended buffer signature -
> and augments audit records with the file hash. (Support for measuring,
> augmenting audit records, and/or verifying fs-verity signatures has
> yet to be added.)
>
> As explained in my response to 11/19, the file descriptor provides the
> file pathname associated with the buffer data.  In addition, IMA
> policy rules may be defined in terms of other file descriptor info -
> uid, euid, uuid, etc.
>
> Recently support was added for measuring the kexec boot command line,
> certificates being loaded onto a keyring, and blacklisted file hashes
> (limited to appended signatures).  None of these buffers are signed.
>  process_buffer_measurement() was added for this reason and as a
> result is limited to just measuring the buffer data.
>
> Whether process_measurement() or process_buffer_measurement() should
> be modified, needs to be determined.  In either case to support the
> init_module syscall, would at minimum require the associated file
> pathname.

Right -- I don't intend to make changes to the init_module() syscall
since it's deprecated, so this hook is more of a "fuller LSM coverage
for old syscalls" addition.

IMA can happily continue to ignore it, which is what I have here, but I
thought I'd at least show what it *might* look like. Perhaps BPF LSM is
a better example.

Does anything need to change for this patch?

--
Kees Cook