Re: [PATCH v4 09/17] LSM: Introduce kernel_post_load_data() hook

From: Mimi Zohar
Date: Thu Aug 06 2020 - 18:00:10 EST

On Wed, 2020-07-29 at 10:58 -0700, Kees Cook wrote:
> There are a few places in the kernel where LSMs would like to have
> visibility into the contents of a kernel buffer that has been loaded or
> read. While security_kernel_post_read_file() (which includes the
> buffer) exists as a pairing for security_kernel_read_file(), no such
> hook exists to pair with security_kernel_load_data().
> Earlier proposals for just using security_kernel_post_read_file() with a
> NULL file argument were rejected (i.e. "file" should always be valid for
> the security_..._file hooks, but it appears at least one case was
> left in the kernel during earlier refactoring. (This will be fixed in
> a subsequent patch.)
> Since not all cases of security_kernel_load_data() can have a single
> contiguous buffer made available to the LSM hook (e.g. kexec image
> segments are separately loaded), there needs to be a way for the LSM to
> reason about its expectations of the hook coverage. In order to handle
> this, add a "contents" argument to the "kernel_load_data" hook that
> indicates if the newly added "kernel_post_load_data" hook will be called
> with the full contents once loaded. That way, LSMs requiring full contents
> can choose to unilaterally reject "kernel_load_data" with contents=false
> (which is effectively the existing hook coverage), but when contents=true
> they can allow it and later evaluate the "kernel_post_load_data" hook
> once the buffer is loaded.
> With this change, LSMs can gain coverage over non-file-backed data loads
> (e.g. init_module(2) and firmware userspace helper), which will happen
> in subsequent patches.
> Additionally prepare IMA to start processing these cases.
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Thanks, Kees. Other than a missing "name" field, it looks good.

The security_kernel_post_load_data hook may be used to verify appended
signatures and to measure the buffer data. Passing the kernel module
( and firmware (fw_name) names is critical at least for