[PATCH 17/17] module: Prevent module removal racing with text_poke()

[Masami Hiramatsu]

On Thu, 17 Jan 2019 18:07:03 +0000
Nadav Amit <namit at vmware.com> wrote:

> > On Jan 16, 2019, at 11:54 PM, Masami Hiramatsu <mhiramat at kernel.org> wrote:
> > 
> > On Wed, 16 Jan 2019 16:32:59 -0800
> > Rick Edgecombe <rick.p.edgecombe at intel.com> wrote:
> > 
> >> From: Nadav Amit <namit at vmware.com>
> >> 
> >> It seems dangerous to allow code modifications to take place
> >> concurrently with module unloading. So take the text_mutex while the
> >> memory of the module is freed.
> > 
> > At that point, since the module itself is removed from module list,
> > it seems no actual harm. Or would you have any concern?
> 
> So it appears that you are right and all the users of text_poke() and
> text_poke_bp() do install module notifiers, and remove the module from their
> internal data structure when they are done (*). As long as they prevent
> text_poke*() to be called concurrently (e.g., using jump_label_lock()),
> everything is fine.
> 
> Having said that, the question is whether you “trust” text_poke*() users to
> do so. text_poke() description does not day explicitly that you need to
> prevent modules from being removed.
> 
> What do you say?

I agreed, but in that case, this is just a fool proof. I think we should
prevent this kind of bug by review, and should comment it on text_poke(),
instead of locking text_mutex.

What I thought was even if we take text_mutex here, such user can modify
the (released) module code right after we exit this section.

Maybe we'd better make text_poke() more smart?

> (*) I am not sure about kgdb, but it probably does not matter much

I think we don't need to care about kgdb. It is a tool which should be able
to shoot your feet and we can not prevent it. Only expert can avoid it. :)

Thank you,

-- 
Masami Hiramatsu <mhiramat at kernel.org>