Re: [PATCH v3 11/19] LSM: Introduce kernel_post_load_data() hook

Kees Cook
2020-07-28T19:41:47Z

On Mon, Jul 27, 2020 at 06:49:11AM -0400, Mimi Zohar wrote:
> On Fri, 2020-07-24 at 14:36 -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@chromium.org>
>
> At least from an IMA perspective, the original
> security_kernel_load_data() hook was defined in order to prevent
> certain syscalls - init_module, kexec_load - and loading firmware via
> sysfs.  The resulting error messages were generic.
>
> Unlike security_kernel_load_data(), security_kernel_post_load_data()
> is meant to be used, but without a file desciptor specific
> information, like the filename associated with the buffer, is missing.
>  Having the filename isn't actually necessary for verifying the
> appended signature, but it is needed for auditing signature
> verification failures and including in the IMA measurement list.

Right -- I'm open to ideas on this, but as it stands, other LSMs (e.g.
BPF LSM) can benefit from the security_kernel_post_load_data() to
examine the contents, etc.

Is there anything that needs to change in this patch?

--
Kees Cook