linux-kernel - Re: [PATCH v2] cpufreq: brcmstb-avs-cpufreq: Fix -Warray-bounds bug

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Message-ID: <202307311610.B1EB796684@keescook>
Date:   Mon, 31 Jul 2023 16:16:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Markus Mayer <mmayer@...adcom.com>,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Florian Fainelli <florian.fainelli@...adcom.com>,
        linux-pm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] cpufreq: brcmstb-avs-cpufreq: Fix -Warray-bounds bug

On Mon, Jul 31, 2023 at 03:07:20PM -0600, Gustavo A. R. Silva wrote:
> Update the iteration conditions in the for() loop to avoid writing in
> array `table` beyond its allocated size at:
> 
> drivers/cpufreq/brcmstb-avs-cpufreq.c:
> 449         table[i].frequency = CPUFREQ_TABLE_END;
> 
> This fixes the following -Warray-bounds warning seen after building
> ARM with multi_v7_defconfig (GCC 13):
> In function 'brcm_avs_get_freq_table',
>     inlined from 'brcm_avs_cpufreq_init' at drivers/cpufreq/brcmstb-avs-cpufreq.c:623:15:
> drivers/cpufreq/brcmstb-avs-cpufreq.c:449:28: warning: array subscript 5 is outside array bounds of 'void[60]' [-Warray-bounds=]
>   449 |         table[i].frequency = CPUFREQ_TABLE_END;

#define AVS_PSTATE_P0           0x0
#define AVS_PSTATE_P1           0x1
#define AVS_PSTATE_P2           0x2
#define AVS_PSTATE_P3           0x3
#define AVS_PSTATE_P4           0x4
#define AVS_PSTATE_MAX          AVS_PSTATE_P4

        table = devm_kcalloc(dev, AVS_PSTATE_MAX + 1, sizeof(*table),
                             GFP_KERNEL);
	...
        for (i = AVS_PSTATE_P0; i <= AVS_PSTATE_MAX; i++) {
		...
	}
        table[i].frequency = CPUFREQ_TABLE_END;

I see "AVS_PSTATE_MAX + 1" being used for the allocation, and so the
loop is likely correctly doing P0 through P4. If there is supposed to be
a terminating element in the table, I think the correct fix would be to
allocate an additional element, not stop the loop from processing P4.

> [...]
> -	for (i = AVS_PSTATE_P0; i <= AVS_PSTATE_MAX; i++) {
> +	for (i = AVS_PSTATE_P0; i < AVS_PSTATE_MAX; i++) {
>  		ret = brcm_avs_set_pstate(priv, i);
>  		if (ret)
>  			return ERR_PTR(ret);

-Kees

-- 
Kees Cook