Re: [PATCH] opp: Fix use-after-free in lazy_opp_tables after probe deferral

[Stephan Gerhold <stephan.gerhold@kernkonzept.com>]

On Mon, May 29, 2023 at 11:01:48AM +0530, Viresh Kumar wrote:
> On 24-05-23, 19:56, Stephan Gerhold wrote:
> > When dev_pm_opp_of_find_icc_paths() in _allocate_opp_table() returns
> > -EPROBE_DEFER, the opp_table is freed again, to wait until all the
> > interconnect paths are available.
> > 
> > However, if the OPP table is using required-opps then it may already
> > have been added to the global lazy_opp_tables list. The error path
> > does not remove the opp_table from the list again.
> > 
> > This can cause crashes later when the provider of the required-opps
> > is added, since we will iterate over OPP tables that have already been
> > freed. E.g.:
> > 
> >   Unable to handle kernel NULL pointer dereference when read
> >   CPU: 0 PID: 7 Comm: kworker/0:0 Not tainted 6.4.0-rc3
> >   PC is at _of_add_opp_table_v2 (include/linux/of.h:949
> >   drivers/opp/of.c:98 drivers/opp/of.c:344 drivers/opp/of.c:404
> >   drivers/opp/of.c:1032) -> lazy_link_required_opp_table()
> > 
> > Fix this by removing the opp_table from the list before freeing it.
> 
> I think you need this instead:
> 
> diff --git a/drivers/opp/core.c b/drivers/opp/core.c
> index 954c94865cf5..b5973fefdfd8 100644
> --- a/drivers/opp/core.c
> +++ b/drivers/opp/core.c
> @@ -1358,7 +1358,10 @@ static struct opp_table *_allocate_opp_table(struct device *dev, int index)
>         return opp_table;
> 
>  remove_opp_dev:
> +       _of_clear_opp_table(opp_table);
>         _remove_opp_dev(opp_dev, opp_table);
> +       mutex_destroy(&opp_table->genpd_virt_dev_lock);
> +       mutex_destroy(&opp_table->lock);
>  err:
>         kfree(opp_table);
>         return ERR_PTR(ret);
> 

Thanks, this seems to fix the crash as well. Are you going to handle it
or should I send a v2 with this diff?

> > Cc: stable@vger.kernel.org
> > Fixes: 7eba0c7641b0 ("opp: Allow lazy-linking of required-opps")
> > Signed-off-by: Stephan Gerhold <stephan.gerhold@kernkonzept.com>
> > ---
> > This fixes the crash I ran into after adding an OPP table with
> > both "required-opps" and interconnect paths (opp-peak-kBps).
> > 
> > By the way, the "lazy_opp_tables" does not seem to be protected by any
> > locks(?)
> 
> It is always accessed with opp_table_lock held I believe.
> 

During _allocate_opp_table() it's accessed without the opp_table_lock,
because of

	/* Drop the lock to reduce the size of critical section */
	mutex_unlock(&opp_table_lock);

	if (opp_table) {
		/* ... */
		mutex_lock(&opp_table_lock);
	} else {
		opp_table = _allocate_opp_table(dev, index);

		mutex_lock(&opp_table_lock);
		/* ... */
	}

This doesn't seem to cause any problems in my case though so it's
unrelated to the crash I observed.

Thanks,
Stephan
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