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

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Message-ID: <87lgd0o1zr.fsf@xmission.com>
Date:   Thu, 03 May 2018 16:36:40 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Mimi Zohar <zohar@...ux.vnet.ibm.com>
Cc:     Casey Schaufler <casey@...aufler-ca.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

Mimi Zohar <zohar@...ux.vnet.ibm.com> writes:

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