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Date: Wed, 24 Apr 2024 16:49:42 -0700
From: Kees Cook <keescook@chromium.org>
To: Helge Deller <deller@gmx.de>
Cc: Justin Stitt <justinstitt@google.com>, linux-fbdev@vger.kernel.org,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH] video: fbdev: au1200fb: replace deprecated strncpy with
 strscpy
Message-ID: <202404241647.E1DE1D9@keescook>
References: <20240318-strncpy-drivers-video-fbdev-au1200fb-c-v1-1-680802a9f10a@google.com>
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 <CAFhGd8ppVq9aGbfFLeL30jQ15KHS=FoLh0c1udXo=Z+pCfXL1g@mail.gmail.com>
 <49ba1e7d-d256-4644-beb9-c84b9feb0052@gmx.de>
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On Wed, Mar 20, 2024 at 11:48:52PM +0100, Helge Deller wrote:
> On 3/20/24 23:35, Justin Stitt wrote:
> > Hi,
> > 
> > On Wed, Mar 20, 2024 at 12:56 AM Helge Deller <deller@gmx.de> wrote:
> > > 
> > > On 3/19/24 00:46, 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.
> > > > 
> > > > Let's use the new 2-argument strscpy() which guarantees NUL-termination
> > > > on the destination buffer while also simplifying the syntax. Note that
> > > > strscpy() will not NUL-pad the destination buffer like strncpy() does.
> > > > 
> > > > However, the NUL-padding behavior of strncpy() is not required since
> > > > fbdev is already NUL-allocated from au1200fb_drv_probe() ->
> > > > frameuffer_alloc(), rendering any additional NUL-padding redundant.
> > > > |     p = kzalloc(fb_info_size + size, GFP_KERNEL);
> > > > 
> > > > 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>
> > > > ---
> > > > Note: build-tested only.
> > > > 
> > > > Found with: $ rg "strncpy\("
> > > > ---
> > > >    drivers/video/fbdev/au1200fb.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/video/fbdev/au1200fb.c b/drivers/video/fbdev/au1200fb.c
> > > > index 6f20efc663d7..e718fea63662 100644
> > > > --- a/drivers/video/fbdev/au1200fb.c
> > > > +++ b/drivers/video/fbdev/au1200fb.c
> > > > @@ -1557,7 +1557,7 @@ static int au1200fb_init_fbinfo(struct au1200fb_device *fbdev)
> > > >                return ret;
> > > >        }
> > > > 
> > > > -     strncpy(fbi->fix.id, "AU1200", sizeof(fbi->fix.id));
> > > > +     strscpy(fbi->fix.id, "AU1200");
> > > 
> > > I wonder if you really build-tested this, as this driver is for the mips architecture...
> > > And I don't see a strscpy() function which takes just 2 arguments.
> > > But I might be wrong....
> > 
> > I did build successfully :thumbs_up:
> > 
> > Commit e6584c3964f2f ("string: Allow 2-argument strscpy()") introduced
> > this new strscpy() form; it is present in string.h on Linus' tree.
> 
> Interesting patch.
> Might give compile problems if patches like yours gets automatically
> picked up to stable series as long as Kees patch hasn't been backported yet...
> Anyway, thanks for the pointer!
> I'll apply your patch in the next round for fbdev.

Hi! I haven't seen this show up in -next yet. Have you had a chance to
pick it up?

There are also these too:

https://lore.kernel.org/all/20240320-strncpy-drivers-video-fbdev-fsl-diu-fb-c-v1-1-3cd3c012fa8c@google.com/
https://patchwork.kernel.org/project/linux-hardening/patch/20240320-strncpy-drivers-video-fbdev-uvesafb-c-v1-1-fd6af3766c80@google.com/
https://patchwork.kernel.org/project/linux-hardening/patch/20240320-strncpy-drivers-video-hdmi-c-v1-1-f9a08168cdaf@google.com/

I can toss all of these into the hardening tree if that makes it easier
for you?

Thanks!

-Kees

-- 
Kees Cook