Re: [PATCH 1/1] staging: ion: Prevent incorrect reference counting behavour

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

On Thu, Nov 25, 2021 at 03:07:37PM +0000, Lee Jones wrote:
> On Thu, 25 Nov 2021, Dan Carpenter wrote:
> 
> > On Thu, Nov 25, 2021 at 02:20:04PM +0000, Lee Jones wrote:
> > > Supply additional checks in order to prevent unexpected results.
> > > 
> > > Fixes: b892bf75b2034 ("ion: Switch ion to use dma-buf")
> > > Signed-off-by: Lee Jones <lee.jones@linaro.org>
> > > ---
> > > Should be back-ported from v4.9 and earlier.
> > > 
> > >  drivers/staging/android/ion/ion.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c
> > > index 806e9b30b9dc8..402b74f5d7e69 100644
> > > --- a/drivers/staging/android/ion/ion.c
> > > +++ b/drivers/staging/android/ion/ion.c
> > > @@ -29,6 +29,7 @@
> > >  #include <linux/export.h>
> > >  #include <linux/mm.h>
> > >  #include <linux/mm_types.h>
> > > +#include <linux/overflow.h>
> > >  #include <linux/rbtree.h>
> > >  #include <linux/slab.h>
> > >  #include <linux/seq_file.h>
> > > @@ -509,6 +510,10 @@ static void *ion_handle_kmap_get(struct ion_handle *handle)
> > >  	void *vaddr;
> > >  
> > >  	if (handle->kmap_cnt) {
> > > +		if (check_add_overflow(handle->kmap_cnt,
> > > +				       (unsigned int) 1, &handle->kmap_cnt))
> >                                                          ^^^^^^^^^^^^^^^^^
> > 
> > > +			return ERR_PTR(-EOVERFLOW);
> > > +
> > >  		handle->kmap_cnt++;
> >                 ^^^^^^^^^^^^^^^^^^^
> > This will not do what you want at all.  It's a double increment on the
> > success path and it leave handle->kmap_cnt overflowed on failure path.
> 
> I read the helper to take copies of the original variables.
> 
> #define __unsigned_add_overflow(a, b, d) ({     \
>         typeof(a) __a = (a);                    \
>         typeof(b) __b = (b);                    \
>         typeof(d) __d = (d);                    \
>         (void) (&__a == &__b);                  \
>         (void) (&__a == __d);                   \
>         *__d = __a + __b;                       \
          ^^^^^^^^^^^^^^^^
This assignment sets handle->kmap_cnt to the overflowed value.

>         *__d < __a;                             \
> })
> 
> Maybe I misread it.
> 
> So the original patch [0] was better?
> 
> [0] https://lore.kernel.org/stable/20211125120234.67987-1-lee.jones@linaro.org/ 

The original at least worked.  :P

You're catching me right as I'm knocking off for the day so I'm not
sure how to write this code.  I had thought that ->kmap_cnt was a
regular int and not an unsigned int, but I would have to pull a stable
tree to see where I misread the code.

I'll look at this tomorrow Nairobi time, but I expect by then you'll
already have it all figured out.

regards,
dan carpenter