linux-kernel - Re: [PATCH 2/3] kexec: call LSM hook for kexec

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Date:   Thu, 03 May 2018 11:42:09 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

Casey Schaufler <casey@...aufler-ca.com> writes:

> On 5/3/2018 8:51 AM, Eric W. Biederman wrote:
>> Mimi Zohar <zohar@...ux.vnet.ibm.com> writes:
>>
>>> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:
>>>> Mimi Zohar <zohar@...ux.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.

Eric