linux-kernel - Re: [PATCH] module: print module name on refcount error

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Message-ID: <ZKuaZepw51Nriqr8@dhcp22.suse.cz>
Date:   Mon, 10 Jul 2023 07:43:01 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Jean Delvare <jdelvare@...e.de>, linux-modules@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] module: print module name on refcount error

On Fri 07-07-23 11:56:49, Luis Chamberlain wrote:
> On Mon, Jul 03, 2023 at 03:47:22PM +0200, Michal Hocko wrote:
> > On Fri 30-06-23 16:05:33, Luis Chamberlain wrote:
> > [...]
> > > What prevents code from racing the free with a random module_put()
> > > called by some other piece of code?
> > 
> > Wouldn't be ref count a garbage already? How can you race when freeing
> > if module_put fail?
> 
> It could yes, ie, so this risks at all being junk.

Could you be more specific please? I still do not see a scenario where
module string name would be junk while refcount itself would be a valid
memory.

> So best IMHO is
> to tidy up all the get / puts and add respective tests to fix all
> this mess with proper messages as needed. My cursory review of the
> refcnt stuf is I see some races possible.

It would likely be better to use refcount_t instead of atomic_t.

> While I'd be happy to help debugging aids, adding accesses to random
> memory for a string seems more risk prone.

If there is really a scenario when module could be unloaded leaving
dangling struct module behind then we have a real problem as this is
exported to userspace IIRC. Not to mention module_get/put calls
modifying memory (UAF).
-- 
Michal Hocko
SUSE Labs