Subject: Re: [PATCH net-next 1/3] mlxsw: Warn about invalid accesses to array fields

From: Ido Schimmel <[email-masked]>
Date: Wed, 3 Jul 2024 15:40:08 +0300

On Tue, Jul 02, 2024 at 09:08:17AM +0200, Przemek Kitszel wrote:
> On 7/1/24 18:41, Petr Machata wrote:
> > A forgotten or buggy variable initialization can cause out-of-bounds access
> > to a register or other item array field. For an overflow, such access would
> > mangle adjacent parts of the register payload. For an underflow, due to all
> > variables being unsigned, the access would likely trample unrelated memory.
> > Since neither is correct, replace these accesses with accesses at the index
> > of 0, and warn about the issue.
> 
> That is not correct either, but indeed better.
> 
> > 
> > Suggested-by: Ido Schimmel <[email-masked]>
> > Signed-off-by: Petr Machata <[email-masked]>
> > Reviewed-by: Ido Schimmel <[email-masked]>
> > ---
> >   drivers/net/ethernet/mellanox/mlxsw/item.h | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/net/ethernet/mellanox/mlxsw/item.h b/drivers/net/ethernet/mellanox/mlxsw/item.h
> > index cfafbeb42586..9f7133735760 100644
> > --- a/drivers/net/ethernet/mellanox/mlxsw/item.h
> > +++ b/drivers/net/ethernet/mellanox/mlxsw/item.h
> > @@ -218,6 +218,8 @@ __mlxsw_item_bit_array_offset(const struct mlxsw_item *item,
> >   	}
> >   	max_index = (item->size.bytes << 3) / item->element_size - 1;
> > +	if (WARN_ON(index > max_index))
> > +		index = 0;
> 
> you have BUG*() calls just above those lines :(
> anyway, WARN_ON_ONCE(), and perhaps you need to print some additional
> data to finally fix this?

The trace should be enough, but more info can be added:

diff --git a/drivers/net/ethernet/mellanox/mlxsw/item.h b/drivers/net/ethernet/mellanox/mlxsw/item.h
index 9f7133735760..a619a0736bd1 100644
--- a/drivers/net/ethernet/mellanox/mlxsw/item.h
+++ b/drivers/net/ethernet/mellanox/mlxsw/item.h
@@ -218,7 +218,9 @@ __mlxsw_item_bit_array_offset(const struct mlxsw_item *item,
        }
 
        max_index = (item->size.bytes << 3) / item->element_size - 1;
-       if (WARN_ON(index > max_index))
+       if (WARN_ONCE(index > max_index,
+                     "name=%s,index=%u,max_index=%u\n", item->name, index,
+                     max_index))
                index = 0;
        be_index = max_index - index;
        offset = be_index * item->element_size >> 3;

Will leave it to Petr to decide what he wants to include there.

> 
> >   	be_index = max_index - index;
> >   	offset = be_index * item->element_size >> 3;
> >   	in_byte_index  = index % (BITS_PER_BYTE / item->element_size);
>