Re: Linux Kernel module notification bug

[Adam Zabrocki]

Hello,

On Mon, Jan 11, 2021 at 03:20:48PM +0100, Jessica Yu wrote:
> +++ Adam Zabrocki [10/01/21 18:54 +0100]:
> > Hello,
> > 
> > I believe that the following commit does introduce a gentle "functionality
> > bug":
> > 
> > "module: delay kobject uevent until after module init call":
> > https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/kernel/module.c?id=38dc717e97153e46375ee21797aa54777e5498f3
> > 
> > The official Linux Kernel API for the kernel module activities notification has
> > been divided based on the readiness 'stage' for such module. We have the
> > following stages:
> > 
> >        MODULE_STATE_LIVE,      /* Normal state. */
> >        MODULE_STATE_COMING,    /* Full formed, running module_init. */
> >        MODULE_STATE_GOING,     /* Going away. */
> >        MODULE_STATE_UNFORMED,  /* Still setting it up. */
> > 
> > LIVE means that the kernel module is correctly running and all initialization
> > work has been already done. Otherwise, we have event 'COMING' or 'UNFORMED'.
> > 
> > In the described commit, creation of the KOBJECT has been moved after invoking
> > a notficiation of the newly formed kernel module state (LIVE). That's somehow
> > inconsistent from my understanding of the kernel modules notifiers logic.
> > Creation of the new objects (like KOBJ) should be done before notification of
> > the stage LIVE is invoked.
> 
> I'm confused. We're not creating any kobjects here. That is all done
> in mod_sysfs_setup(), which is called while the module is still
> COMING.  What that commit does is delay telling userspace about the
> module (specifically, systemd/udev) until the module is basically
> ready. Systemd was basically receiving the uevent too early, before
> the module has initialized, hence we decided to delay the uevent [1].
> 

Sorry for the confusion on my side. I was referring to the internal state of 
the KOBJ itself which is being actively modified when uevent is sent. During 
the module creation KOBJ_ADD modifies 'kobj->state_add_uevent_sent'. Until 
recent commit, kernel didn't modify KOBJ after sending LIVE notification.

> > This commit breaks some of the projects that rely on the LIVE notification to
> > monitor when the newly loaded module is ready.
> 
> Hm, could you please explain specifically what is the issue you're seeing?
> What projects is it breaking?
> 

I'm specifically referencing these projects which are tracking kernel modules
for integrity purpose (e.g. anti-rootkit tools) like Linux Kernel Runtime 
Guard.
It is possible to modify these tools to adopt and take into account 
modification of KOBJ after LIVE state notification. However, from my 
understanding of the kernel modules notifiers logic, KOBJ should be fully 
formed at this stage.

Thanks,
Adam

> Thanks,
> 
> Jessica
> 
> [1] https://github.com/systemd/systemd/issues/17586

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