Re: [PATCH v1] RDMA/nldev: Add length check for IFLA_BOND_ARP_IP_TARGET parsing

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 25 Jul 2023 08:25:57 +0300 Leon Romanovsky wrote:
> > Yeah I have seen that. Just as Jakub said, empty netlink attributes are valid 
> > (they are viewed as flag). The point is that different attribute has different
> > length requirement. For this specific code, the RDMA_NLDEV_ATTR_STAT_HWCOUNTERS
> > attribute is a nested one whose inner attributes should be NLA_U32. But as you
> > can see in variable nldev_policy, the description does not use nested policy to
> > enfore that, which results in the bug discussed in my commit message.
> > 
> >  [RDMA_NLDEV_ATTR_STAT_HWCOUNTERS]       = { .type = NLA_NESTED },
> > 
> > The elegant fix could be add the nested policy description to nldev_policy while
> > this is toublesome as no existing nla_attr has been given to this nested nlattr.
> > Hence, add the length check is the simplest solution and you can see such nla_len
> > check code all over the kernel.  
> 
> Right, and this is what bothers me.
> 
> I would more than happy to change nla_for_each_nested() to be something
> like nla_for_each_nested_type(...., sizeof(u32)), which will skip empty
> lines, for code which can't have them.

In general the idea of auto-skipping stuff kernel doesn't recognize
is a bit old school. Better direction would be extending the policy
validation to cover use cases for such loops.