linux-hardening - Re: [PATCH v2] power: supply: surface_battery: replace deprecated strncpy with strscpy

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Date: Sat, 21 Oct 2023 10:45:05 +0200
From: Maximilian Luz <luzmaximilian@...il.com>
To: Justin Stitt <justinstitt@...gle.com>, Sebastian Reichel <sre@...nel.org>
Cc: linux-pm@...r.kernel.org, platform-driver-x86@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] power: supply: surface_battery: replace deprecated
 strncpy with strscpy

On 10/20/23 21:39, Justin Stitt wrote:
> strncpy() is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous string
> interfaces.
> 
> We expect bat->name to be NUL-terminated based on its usage with
> strcmp():
> 
> power_supply_core.c:
> 445: return strcmp(psy->desc->name, name) == 0;
> 
> ... and also by the manual `... - 1` for the length argument of the
> original strncpy() invocation.
> 
> Furthermore, no NUL-padding is needed as bat is zero-allocated before
> calling spwr_battery_init():
> 826: bat = devm_kzalloc(&sdev->dev, sizeof(*bat), GFP_KERNEL);
> 827: if (!bat)
> 828:   return -ENOMEM;
> 829:
> 830: spwr_battery_init(bat, sdev, p->registry, p->name);
> 
> ... this means any further NUL-byte assignments (like the ones that
> strncpy() does) are redundant.
> 
> 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.
> 
> Let's also opt to use the more idiomatic strscpy() usage of:
> (dest, src, sizeof(dest)).
> 
> 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>

Thanks!

Reviewed-by: Maximilian Luz <luzmaximilian@...il.com>
Tested-by: Maximilian Luz <luzmaximilian@...il.com>

> ---
> Changes in v2:
> - fix subject line
> - Link to v1: https://lore.kernel.org/r/20231020-strncpy-drivers-power-supply-surface_battery-c-v1-1-cabaea50e667@google.com
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>   drivers/power/supply/surface_battery.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/power/supply/surface_battery.c b/drivers/power/supply/surface_battery.c
> index 19d2f8834e56..196d290dc596 100644
> --- a/drivers/power/supply/surface_battery.c
> +++ b/drivers/power/supply/surface_battery.c
> @@ -722,7 +722,7 @@ static void spwr_battery_init(struct spwr_battery_device *bat, struct ssam_devic
>   			      struct ssam_event_registry registry, const char *name)
>   {
>   	mutex_init(&bat->lock);
> -	strncpy(bat->name, name, ARRAY_SIZE(bat->name) - 1);
> +	strscpy(bat->name, name, sizeof(bat->name));
>   
>   	bat->sdev = sdev;
>   
> 
> ---
> base-commit: bb55d7f7f7445abcc8db50e6a65d4315e79f75c7
> change-id: 20231020-strncpy-drivers-power-supply-surface_battery-c-b0c84b05ac28
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
>