linux-kernel - [PATCH] module: invoke kobject uevent before sending LIVE notification

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Date:   Wed, 13 Jan 2021 01:33:10 +0100
From:   Adam Zabrocki <pi3@....com.pl>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Nicolas Morey-Chaisemartin <nmoreychaisemartin@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, Solar Designer <solar@...nwall.com>
Subject: [PATCH] module: invoke kobject uevent before sending LIVE
 notification

The recent change "module: delay kobject uevent until after module init
call", while helping avoid a race between udev/systemd and the module
loader, made it unnecessarily more difficult to monitor kernel module
integrity by out-of-tree projects such as Linux Kernel Runtime Guard.

Specifically, that change delayed the kobject uevent unnecessarily too far,
to until after sending a MODULE_STATE_LIVE notification.  As the uevent
modifies internal state of the KOBJ itself, this violated the assumption
(non-guaranteed yet handy while we can maintain it) that the KOBJ remains
consistent and can be integrity-checked as soon as the module is LIVE.

To make all of these projects happy at once, move the kobject KOBJ_ADD
uevent to just before sending the MODULE_STATE_LIVE notification.

Fixes: 38dc717e9715 ("module: delay kobject uevent until after module init call")
Signed-off-by: Adam Zabrocki <pi3@....com.pl>
---
 kernel/module.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 4bf30e4b3eaa..7d56b1b07237 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3681,14 +3681,14 @@ static noinline int do_init_module(struct module *mod)
                dump_stack();
        }

+       /* Delay uevent until module has finished its init routine */
+       kobject_uevent(&mod->mkobj.kobj, KOBJ_ADD);
+
        /* Now it's a first class citizen! */
        mod->state = MODULE_STATE_LIVE;
        blocking_notifier_call_chain(&module_notify_list,
                                     MODULE_STATE_LIVE, mod);

-       /* Delay uevent until module has finished its init routine */
-       kobject_uevent(&mod->mkobj.kobj, KOBJ_ADD);
-
        /*
         * We need to finish all async code before the module init sequence
         * is done.  This has potential to deadlock.  For example, a newly