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Date:   Mon, 18 Sep 2023 09:16:39 +0200
From:   Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     Justin Stitt <justinstitt@google.com>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] drm/gma500: refactor deprecated strncpy
Message-ID: <7rldvf22zvtvvqpqsoeidg57sqwnuz6pfclve7igw6kuxp5jfp@vkt3z7br5hll>
Mail-Followup-To: Kees Cook <keescook@chromium.org>, 
        Justin Stitt <justinstitt@google.com>, David Airlie <airlied@gmail.com>, 
        Daniel Vetter <daniel@ffwll.ch>, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, 
        linux-hardening@vger.kernel.org
References: <20230914-drivers-gpu-drm-gma500-oaktrail_lvds_i2c-c-v1-1-0a53a076ce20@google.com>
 <202309142135.16032DEA8@keescook>
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On Thu, Sep 14, 2023 at 09:37:31PM -0700, Kees Cook wrote:
> 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@vger.kernel.org
> > Signed-off-by: Justin Stitt <justinstitt@google.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.

It seems I2C_NAME_SIZE is used for i2c_client.name but is abused here
for i2c_adapter.name as well. Using sizeof() would be better. Justin,
would you mind changing that as well?

Thanks
Patrik

> 
> Ultimately it doesn't matter since the source is a const char string,
> but it's still weird. Therefore:
> 
> Reviewed-by: Kees Cook <keescook@chromium.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@google.com>
> > 
> 
> -- 
> Kees Cook