linux-hardening - Re: [PATCH] iio: adc: stm32-adc: replace deprecated strncpy

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Date: Sat, 23 Sep 2023 20:15:09 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Jonathan Cameron <jic23@...nel.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	linux-iio@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] iio: adc: stm32-adc: replace deprecated strncpy

On Thu, Sep 21, 2023 at 04:54:00AM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> We should prefer more robust and less ambiguous string interfaces.
> 
> We expect adc->chan_name[val] to be NUL-terminated based on ch_name's
> use within functions that expect NUL-terminated strings like strncmp and
> printf-likes:
> | 	if (!strncmp(stm32_adc_ic[i].name, ch_name, STM32_ADC_CH_SZ)) {
> | 		/* Check internal channel availability */
> | 		switch (i) {
> | 		case STM32_ADC_INT_CH_VDDCORE:
> | 			if (!adc->cfg->regs->or_vddcore.reg)
> | 				dev_warn(&indio_dev->dev,
> | 					 "%s channel not available\n", ch_name);
> ...
> 
> There is no evidence that NUL-padding is needed either.

Agreed -- it's used as a C string everywhere I can see.

> 
> Considering the above, a suitable replacement is `strscpy` [2] due to
> the fact that it guarantees NUL-termination on the destination buffer
> without unnecessarily NUL-padding. If, for any reason, NUL-padding _is_
> required we should go for `strscpy_pad`.
> 
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note: build-tested
> ---
>  drivers/iio/adc/stm32-adc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/adc/stm32-adc.c b/drivers/iio/adc/stm32-adc.c
> index f7613efb870d..9cdcf396d901 100644
> --- a/drivers/iio/adc/stm32-adc.c
> +++ b/drivers/iio/adc/stm32-adc.c
> @@ -2209,7 +2209,7 @@ static int stm32_adc_generic_chan_init(struct iio_dev *indio_dev,
>  				ret = -EINVAL;
>  				goto err;
>  			}
> -			strncpy(adc->chan_name[val], name, STM32_ADC_CH_SZ);
> +			strscpy(adc->chan_name[val], name, STM32_ADC_CH_SZ);

I still prefer sizeof($dst), but yes, these are the same:

        char chan_name[STM32_ADC_CH_MAX][STM32_ADC_CH_SZ];

If this needs a v2, please improve the Subject, but it is technically
correct, so:

Reviewed-by: Kees Cook <keescook@...omium.org>

:)

-Kees

-- 
Kees Cook