Re: [PATCH net-next] net: ethtool: Fix suspicious rcu_dereference usage

[Eric Dumazet]

On Tue, Dec 17, 2024 at 3:03 PM Kory Maincent <kory.maincent@xxxxxxxxxxx> wrote:
>
> The __ethtool_get_ts_info function can be called with or without the
> rtnl lock held. When the rtnl lock is not held, using rtnl_dereference()
> triggers a warning due to improper lock context.
>
> Replace rtnl_dereference() with rcu_dereference_rtnl() to safely
> dereference the RCU pointer in both scenarios, ensuring proper handling
> regardless of the rtnl lock state.
>
> Reported-by: syzbot+a344326c05c98ba19682@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/netdev/676147f8.050a0220.37aaf.0154.GAE@xxxxxxxxxx/
> Fixes: b9e3f7dc9ed9 ("net: ethtool: tsinfo: Enhance tsinfo to support several hwtstamp by net topology")
> Signed-off-by: Kory Maincent <kory.maincent@xxxxxxxxxxx>
> ---
>  net/ethtool/common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ethtool/common.c b/net/ethtool/common.c
> index 02f941f667dd..ec6f2e2caaf9 100644
> --- a/net/ethtool/common.c
> +++ b/net/ethtool/common.c
> @@ -870,7 +870,7 @@ int __ethtool_get_ts_info(struct net_device *dev,
>  {
>         struct hwtstamp_provider *hwprov;
>
> -       hwprov = rtnl_dereference(dev->hwprov);
> +       hwprov = rcu_dereference_rtnl(dev->hwprov);
>         /* No provider specified, use default behavior */
>         if (!hwprov) {
>                 const struct ethtool_ops *ops = dev->ethtool_ops;

I have to ask : Can you tell how this patch has been tested ?

rcu is not held according to syzbot report.

If rtnl and rcu are not held, lockdep will still complain.