Re: drivers/xen/gntdev.c:817 gntdev_get_page() warn: mask and shift to zero: expr='(addr & ~(~((1 << 12) - 1))) >> 12'

[Dan Carpenter <error27@gmail.com>]

On Fri, Sep 25, 2026 at 08:43:54AM +0200, Arnd Bergmann wrote:
> On Thu, Sep 24, 2026, at 21:39, Dan Carpenter wrote:
> 
> >    489                          return -EFAULT;
> >    490                  break;
> >    491          case 2:
> >    492                  if (copy_from_user(&m, udata, sizeof(struct 
> > privcmd_mmapbatch_v2)))
> >    493                          return -EFAULT;
> >    494                  /* Returns per-frame error code in m.err. */
> >    495                  if (!access_ok(m.err, m.num * (sizeof(*m.err))))
> >                                               ^^^^^^^^^^^^^^^^^^^^^^^
> > These integer overflow bugs are from 2012, but I guess your patch 
> > exposed
> > the arm32 build to the zero day bot.  The bugs only affect 32bit 
> > systems.
> 
> Right, the randconfig came up with an ARMv6 Xen build, which was not
> possible before my patch. I'm sure this was reported for other configs
> before and just showed up as introduced by my patch here.
> 
> This is clearly a bug but it does look harmless to me, as it only
> results in the userspace corrupting itself when passing invalid
> data.

In ancient times, these access_ok() overflows were a much bigger deal.
Easy to solve with a size_mul(m.num, sizeof(*m.err)).

regards,
dan carpenter