Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

Mimi Zohar
2018-05-03T21:07:11Z

On Thu, 2018-05-03 at 11:42 -0500, Eric W. Biederman wrote:
> Casey Schaufler <casey@schaufler-ca.com> writes:
>
> > On 5/3/2018 8:51 AM, Eric W. Biederman wrote:
> >> Mimi Zohar <zohar@linux.vnet.ibm.com> writes:
> >>
> >>> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:
> >>>> Mimi Zohar <zohar@linux.vnet.ibm.com> writes:
> >>>>
> >>>>> Allow LSMs and IMA to differentiate between the kexec_load and
> >>>>> kexec_file_load syscalls by adding an "unnecessary" call to
> >>>>> security_kernel_read_file() in kexec_load. This would be similar to the
> >>>>> existing init_module syscall calling security_kernel_read_file().
> >>>> Given the reasonable desire to load a policy that ensures everything
> >>>> has a signature I don't have fundamental objections.
> >>>>
> >>>> security_kernel_read_file as a hook seems an odd choice. At the very
> >>>> least it has a bad name because there is no file reading going on here.
> >>>>
> >>>> I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested
> >>>> anywhere. Which means I could have a kernel compiled without that and I
> >>>> would be allowed to use kexec_file_load without signature checking.
> >>>> While kexec_load would be denied.
> >>>>
> >>>> Am I missing something here?
> >>> The kexec_file_load() calls kernel_read_file_from_fd(), which in turn
> >>> calls security_kernel_read_file().  So kexec_file_load and kexec_load
> >>> syscall would be using the same method for enforcing signature
> >>> verification.
> >> Having looked at your patches and the kernel a little more I think
> >> this should be a separate security hook that does not take a file
> >> parameter.
> >>
> >> Right now every other security module assumes !file is init_module.
> >> So I think this change has the potential to confuse other security
> >> modules, with the result of unintended policy being applied.
> >>
> >> So just for good security module hygeine I think this needs a dedicated
> >> kexec_load security hook.
> >
> > I would rather see the existing modules updated than a new
> > hook added. Too many hooks spoil the broth. Two hooks with
> > trivial differences just add to the clutter and make it harder
> > for non-lsm developers to figure out what to use in their
> > code.
>
> These are not non-trivial differences. There is absolutely nothing
> file related about kexec_load. Nor for init_module for that matter.
>
> If something is called security_kernel_read_file I think it is wholly
> appropriate for code that processes such a hook to assume file is
> non-NULL.
>
> When you have to dance a jig (which is what I see the security modules
> doing) to figure out who is calling a lsm hook for what purpose I think
> it is a maintenance problem waiting to happen and that the hook is badly
> designed.
>
> At this point I don't care what the lsm's do with the hooks but the
> hooks need to make sense for people outside of the lsm's and something
> about reading a file in a syscall that doesn't read files is complete
> and utter nonsense.

Sure, we can define a wrapper around the security_kernel_read_file()
hook, calling it security_non-fd_syscall() or even
security_old_syscall().

Mimi