linux-kernel - Re: [PATCH v2 3/4] cpufreq: mediatek: add Mediatek cpufreq driver

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Date:	Mon, 9 Mar 2015 16:28:09 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	"pi-cheng.chen" <pi-cheng.chen@...aro.org>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org, linaro-kernel@...ts.linaro.org,
	Mike Turquette <mturquette@...aro.org>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-pm@...r.kernel.org, Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, fan.chen@...iatek.com,
	Howard Chen <ibanezchen@...il.com>,
	Ashwin Chaugule <ashwin.chaugule@...aro.org>,
	linux-mediatek@...ts.infradead.org,
	Kumar Gala <galak@...eaurora.org>,
	"Joe.C" <yingjoe.chen@...iatek.com>,
	Eddie Huang <eddie.huang@...iatek.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 3/4] cpufreq: mediatek: add Mediatek cpufreq driver

On Wed, Mar 04, 2015 at 04:49:15PM +0800, pi-cheng.chen wrote:
> +static int cpu_opp_table_get_freq_index(unsigned int freq)
> +{
> +	struct cpu_opp_table *opp_tbl = dvfs_info->opp_tbl;
> +	int i;
> +
> +	for (i = 0; opp_tbl[i].freq != 0; i++) {
> +		if (opp_tbl[i].freq >= freq)
> +			return i;
> +	}
> +
> +	return -1;

My "return -1" detector fired on this...

> +static int cpu_opp_table_get_volt_index(unsigned int volt)
> +{
> +	struct cpu_opp_table *opp_tbl = dvfs_info->opp_tbl;
> +	int i;
> +
> +	for (i = 0; opp_tbl[i].vproc != -1; i++)
> +		if (opp_tbl[i].vproc >= volt)
> +			return i;
> +
> +	return -1;

And this.

> +static int mtk_cpufreq_notify(struct notifier_block *nb,
> +			      unsigned long action, void *data)
> +{
> +	struct cpufreq_freqs *freqs = data;
> +	struct cpu_opp_table *opp_tbl = dvfs_info->opp_tbl;
> +	int old_vproc, new_vproc, old_index, new_index;
> +
> +	if (!cpumask_test_cpu(freqs->cpu, &dvfs_info->cpus))
> +		return NOTIFY_DONE;
> +
> +	old_vproc = regulator_get_voltage(dvfs_info->proc_reg);
> +	old_index = cpu_opp_table_get_volt_index(old_vproc);
> +	new_index = cpu_opp_table_get_freq_index(freqs->new * 1000);
> +	new_vproc = opp_tbl[new_index].vproc;

Let's say that cpu_opp_table_get_freq_index() returns -1.  We then
do no error checking on this, and access the memory immediately
preceding opp_tbl[0].

Since we'll be loading garbage from opp_tbl[-1], this probably means
that mtk_cpufreq_voltage_trace() will go wrong.  Your method of using
the vproc values to work out which direction we should walk between
old_index...new_index means that we could end up walking through
almost the whole UINT_MAX range to wrap around to the new index.

Yet again, "return -1" proves to be a sure sign of a bug.

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