linux-hardening - Re: [PATCH] drm/gma500: refactor deprecated strncpy

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Date:   Thu, 14 Sep 2023 21:37:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Patrik Jakobsson <patrik.r.jakobsson@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] drm/gma500: refactor deprecated strncpy

On Thu, Sep 14, 2023 at 08:52:21PM +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.
> 
> Since `chan->base.name` is expected to be NUL-terminated, a suitable
> replacement is `strscpy` [2] due to the fact that it guarantees
> NUL-termination on the destination buffer without also unnecessarily
> NUL-padding.

How did you decide about %NUL padding? (I see it is kzalloc'd, so it
doesn't matter.)

> 
> 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>
> ---
> 
> 
> drm/gma500: refactor deprecated strncpy
> ---
>  drivers/gpu/drm/gma500/oaktrail_lvds_i2c.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/gma500/oaktrail_lvds_i2c.c b/drivers/gpu/drm/gma500/oaktrail_lvds_i2c.c
> index 06b5b2d70d48..68458cbdd6d5 100644
> --- a/drivers/gpu/drm/gma500/oaktrail_lvds_i2c.c
> +++ b/drivers/gpu/drm/gma500/oaktrail_lvds_i2c.c
> @@ -141,7 +141,7 @@ struct gma_i2c_chan *oaktrail_lvds_i2c_init(struct drm_device *dev)
>  
>  	chan->drm_dev = dev;
>  	chan->reg = dev_priv->lpc_gpio_base;
> -	strncpy(chan->base.name, "gma500 LPC",  I2C_NAME_SIZE - 1);
> +	strscpy(chan->base.name, "gma500 LPC",  I2C_NAME_SIZE - 1);

What's going on here with the destination buffer size? chan->base.name
is 48 bytes. I2C_NAME_SIZE is 20.

Ultimately it doesn't matter since the source is a const char string,
but it's still weird. Therefore:

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

-Kees

>  	chan->base.owner = THIS_MODULE;
>  	chan->base.algo_data = &chan->algo;
>  	chan->base.dev.parent = dev->dev;
> 
> ---
> base-commit: 3669558bdf354cd352be955ef2764cde6a9bf5ec
> change-id: 20230914-drivers-gpu-drm-gma500-oaktrail_lvds_i2c-c-a53c6d8bd62f
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 

-- 
Kees Cook