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

Mimi Zohar [off-list ref] writes:

On Thu, 2018-05-03 at 11:42 -0500, Eric W. Biederman wrote:

quoted

Casey Schaufler [off-list ref] writes:

quoted

On 5/3/2018 8:51 AM, Eric W. Biederman wrote:

quoted

Mimi Zohar [off-list ref] writes:

quoted

On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:

quoted

Mimi Zohar [off-list ref] writes:

quoted

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().

I really don't see why you want to use the same hook.

I just read through the code of all three users.  None of them.
Especially IMA shares any significant code between the !file case and
the file case.

Eric

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