linux-kernel - Re: [PATCH v5 1/1] security: Add mechanism to safely (un)load LSMs after boot time

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Date:   Mon, 09 Apr 2018 14:25:01 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     LSM <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        James Morris <jmorris@...ei.org>,
        Peter Dolding <oiaohm@...il.com>,
        Igor Stoppa <igor.stoppa@...wei.com>
Subject: Re: [PATCH v5 1/1] security: Add mechanism to safely (un)load LSMs after
 boot time

Sargun Dhillon wrote:
> >   Remove SECURITY_HOOK_COUNT and "struct security_hook_list"->owner and
> >   the exception in randomize_layout_plugin.c because preventing module
> >   unloading won't work as expected.
> >
> 
> Rather than completely removing the unloading code, might it make
> sense to add a BUG_ON or WARN_ON, in security_delete_hooks if
> allow_unload_module is false, and owner is not NULL?

Do we need to check ->owner != NULL? Although it will be true that
SELinux's ->owner == NULL and LKM-based LSM module's ->owner != NULL,
I think we unregister SELinux before setting allow_unload_module to false.
Thus, rejecting delete_security_hooks() if allow_unload_module == false will
be sufficient. SELinux might want to call panic() if delete_security_hooks()
did not unregister due to allow_unload_module == false. Also,
allow_unload_module would be renamed to allow_unregister_module.

By the way, please don't use BUG_ON() or WARN_ON() because syzbot would hit
and call panic() because syzbot runs tests with panic_on_warn == true.