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Date:   Tue, 1 Aug 2023 11:15:40 +0300
From:   Leon Romanovsky <leon@kernel.org>
To:     Lin Ma <linma@zju.edu.cn>
Cc:     Jakub Kicinski <kuba@kernel.org>, jgg@ziepe.ca,
        markzhang@nvidia.com, michaelgur@nvidia.com, ohartoov@nvidia.com,
        chenzhongjin@huawei.com, yuancan@huawei.com,
        linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1] RDMA/nldev: Add length check for
 IFLA_BOND_ARP_IP_TARGET parsing
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On Mon, Jul 31, 2023 at 08:33:02PM +0800, Lin Ma wrote:
> Hello there,
> 
> > > > > 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.
> > 
> > I'm all in for any solution which will help for average developer to write
> > netlink code without mistakes.
> > 
> > Thanks
> 
> I have just come out a new solution for such length issues. Please see
> * https://lore.kernel.org/all/20230731121247.3972783-1-linma@zju.edu.cn/T/#u
> * https://lore.kernel.org/all/20230731121324.3973136-1-linma@zju.edu.cn/T/#u
> 
> I'm not sure adding additional validation logic in the main nlattr code is
> the best solution. Still, after investigating the code, the len field can
> be very suitable for handling the NLA_NESTED cases here. And the developer
> can do manual parsing with better nla_policy-based checking too.
> 
> If this idea is applied, I will also write a script to clean up other
> nla_len patches based on the nla_policy check.

It looks like Jakub didn't like the idea and we will need to add your
sizeof checks all other the place.

Thanks

> 
> Regards
> Lin