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

[Dan Carpenter <dan.carpenter@oracle.com>]

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.

regards,
dan carpenter