On Thu, 2018-05-03 at 11:42 -0500, Eric W. Biederman wrote: > Casey Schaufler <casey@xxxxxxxxxxxxxxxx> writes: > > > On 5/3/2018 8:51 AM, Eric W. Biederman wrote: > >> Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> writes: > >> > >>> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote: > >>>> Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> 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
