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

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

On Mon, Sep 19, 2022 at 11:22:41PM -0700, Hyunwoo Kim wrote:
> 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.


No no.  No need to appologize or resend anything.  This is just
information to help in future research.

regards,
dan carpenter