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

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Date:	Wed, 13 May 2015 02:22:50 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:	"Grygorii.Strashko@...aro.org" <grygorii.strashko@...aro.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	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 Tuesday, May 12, 2015 11:07:33 AM Dmitry Torokhov wrote:
> 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.

Oh well.  Shouldn't we actually do:

int __clk_get(struct clk *clk)
{
 	struct clk_core *core = IS_ERR_OR_NULL(clk) ? NULL : clk->core;

and remove the check from __pm_clk_add() at the same time?

Knowingly crashing on an error encoded as a pointer is kind of disgusting to me
and the difference between that and a random invalid pointer is that poeple who
pass error values encoded as pointers up the stack usually expect them to be
handled cleanly.


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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