Re: [PATCH] pxa3xx-gcu: Fix integer overflow in pxa3xx_gcu_write

From: Hyunwoo Kim <imv4bel@gmail.com>
To: Dan Carpenter <dan.carpenter@oracle.com>
Cc: deller@gmx.de, linux-fbdev@vger.kernel.org,
	Masami Ichikawa <masami.ichikawa@miraclelinux.com>,
	cip-dev <cip-dev@lists.cip-project.org>,
	Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Subject: Re: [PATCH] pxa3xx-gcu: Fix integer overflow in pxa3xx_gcu_write
Date: Mon, 19 Sep 2022 23:22:41 -0700	[thread overview]
Message-ID: <20220920062241.GA321122@ubuntu> (raw)
In-Reply-To: <YylaC1wHHyLw22D3@kadam>

On Tue, Sep 20, 2022 at 09:13:31AM +0300, Dan Carpenter wrote:
> On Tue, Sep 20, 2022 at 09:02:34AM +0300, Dan Carpenter wrote:
> > On Mon, Jun 20, 2022 at 07:00:10AM -0700, Hyunwoo Kim wrote:
> > > In pxa3xx_gcu_write, a count parameter of
> > > type size_t is passed to words of type int.
> > > Then, copy_from_user may cause a heap overflow because
> > > it is used as the third argument of copy_from_user.
> > > 
> > > Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
> > > ---
> > >  drivers/video/fbdev/pxa3xx-gcu.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/video/fbdev/pxa3xx-gcu.c b/drivers/video/fbdev/pxa3xx-gcu.c
> > > index 043cc8f9ef1c..c3cd1e1cc01b 100644
> > > --- a/drivers/video/fbdev/pxa3xx-gcu.c
> > > +++ b/drivers/video/fbdev/pxa3xx-gcu.c
> > > @@ -381,7 +381,7 @@ pxa3xx_gcu_write(struct file *file, const char *buff,
> > >  	struct pxa3xx_gcu_batch	*buffer;
> > >  	struct pxa3xx_gcu_priv *priv = to_pxa3xx_gcu_priv(file);
> > > 
> > > -	int words = count / 4;
> > > +	size_t words = count / 4;
> > 
> > The count variable is actually capped at MAX_RW_COUNT in vfs_write()
> > so "words" cannot be negative.  This patch helps clean up the code but
> > it does not affect run time.
> 
> Btw, the other thing which prevents this from being expliotable is that
> if you pass a negative value to copy_from_user() it will not copy
> anything because of the check in check_copy_size().  See commit
> 6d13de1489b6 ("uaccess: disallow > INT_MAX copy sizes").
> 
> Linus has sort of gotten annoyed with me before for pointing this stuff
> out because it seemed like maybe I wasn't properly grateful to people
> auditing the code and fixing bugs.  I am grateful.  This patch is
> totally the correct thing to do.  It's just that it's not really
> exploitable as described in the commit message.

I found the code that might have the vulnerability, and submitted a patch without actually debugging it.
This is entirely my fault. sorry.

Should I submit a fix patch that fixes the commit message?

Sorry again.

Regards,
Hyunwoo Kim.