linux-kernel - Re: [PATCH] PM / clock_ops: Fix clock error check in __pm_clk

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Message-ID: <20150512180733.GB20725@dtor-ws>
Date:	Tue, 12 May 2015 11:07:33 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	"Grygorii.Strashko@...aro.org" <grygorii.strashko@...aro.org>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Kevin Hilman <khilman@...aro.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM / clock_ops: Fix clock error check in __pm_clk_add()

On Tue, May 12, 2015 at 08:59:03PM +0300, Grygorii.Strashko@...aro.org wrote:
> On 05/12/2015 07:42 PM, Dmitry Torokhov wrote:
> > On Tue, May 12, 2015 at 04:55:39PM +0300, Grygorii.Strashko@...aro.org wrote:
> >> On 05/09/2015 12:05 AM, Dmitry Torokhov wrote:
> >>> On Fri, May 08, 2015 at 10:59:04PM +0200, Geert Uytterhoeven wrote:
> >>>> On Fri, May 8, 2015 at 7:19 PM, Dmitry Torokhov
> >>>> <dmitry.torokhov@...il.com> wrote:
> >>>>> On Fri, May 08, 2015 at 10:47:43AM +0200, Geert Uytterhoeven wrote:
> >>>>>> In the final iteration of commit 245bd6f6af8a62a2 ("PM / clock_ops: Add
> >>>>>> pm_clk_add_clk()"), a refcount increment was added by Grygorii Strashko.
> >>>>>> However, the accompanying IS_ERR() check operates on the wrong clock
> >>>>>> pointer, which is always zero at this point, i.e. not an error.
> >>>>>> This may lead to a NULL pointer dereference later, when __clk_get()
> >>>>>> tries to dereference an error pointer.
> >>>>>>
> >>>>>> Check the passed clock pointer instead to fix this.
> >>>>>
> >>>>> Frankly I would remove the check altogether. Why do we only check for
> >>>>> IS_ERR and not NULL or otherwise validate the pointer? The clk is passed
> >>>>
> >>>> __clk_get() does the NULL check.
> >>>
> >>> No, not really. It _handles_ clk being NULL and returns "everything is
> >>> fine". In any case it is __clk_get's decision what to do.
> >>>
> >>> I dislike gratuitous checks of arguments passed in. Instead of relying
> >>> on APIs refusing grabage we better not pass garbage to these APIs in the
> >>> first place. So I'd change it to trust that we are given a usable
> >>> pointer and simply do:
> >>>
> >>> 	if (!__clk_get(clk)) {
> >>> 		kfree(ce);
> >>> 		return -ENOENTl
> >>> 	}
> >>
> >> Not sure this is right thing to do, because this API initially
> >> was intended to be used as below [1]:
> >> 	clk = of_clk_get(dev->of_node, i));
> >>   	ret = pm_clk_add_clk(dev, clk);
> >>   	clk_put(clk);
> >>
> >> and of_clk_get may return ERR_PTR().
> > 
> > Jeez, that sequence was not meant to be taken literally, it does miss
> > error handling completely. If you notice the majority of users of this
> > API do something like below:
> > 
> > 	i = 0;
> > 	while ((clk = of_clk_get(dev->of_node, i++)) && !IS_ERR(clk)) {
> > 		dev_dbg(dev, "adding clock '%s' to list of PM clocks\n",
> > 			__clk_get_name(clk));
> > 		error = pm_clk_add_clk(dev, clk);
> > 		clk_put(clk);
> > 		if (error) {
> > 			dev_err(dev, "pm_clk_add_clk failed %d\n", error);
> > 			pm_clk_destroy(dev);
> > 			return error;
> > 		}
> > 	}
> > 
> > i.e. it already validates clk pointer before passing it on since it
> > needs to know when to stop iterating.
> 
> np. It's just my opinion - if you agree that code will just crash
> in case of passing invalid @clk argument (in worst case:)
> 
> int __clk_get(struct clk *clk)
> {
> 	struct clk_core *core = !clk ? NULL : clk->core;
> 						^^^ here

Yes, it will crash if you pass invalid pointer here, be it
ERR_PTR-encoded value, or, for example, 0x1, or maybe (void
*)random_32(). The latter will probably not crash right away, but cause
some random damage that will manifest later.

Thanks.

-- 
Dmitry
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