net: ethtool: phy: Check the req_info.pdn field for GET commands

When processing the netlink GET requests to get PHY info, the req_info.pdn
pointer is NULL when no PHY matches the requested parameters, such as when
the phy_index is invalid, or there's simply no PHY attached to the
interface.

Therefore, check the req_info.pdn pointer for NULL instead of
dereferencing it.

Suggested-by: Eric Dumazet <edumazet@google.com>
Reported-by: Eric Dumazet <edumazet@google.com>
Closes: https://lore.kernel.org/netdev/CANn89iKRW0WpGAh1tKqY345D8WkYCPm3Y9ym--Si42JZrQAu1g@mail.gmail.com/T/#mfced87d607d18ea32b3b4934dfa18d7b36669285
Fixes: 17194be ("net: ethtool: Introduce a command to list PHYs on an interface")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20240910174636.857352-1-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: minimaxwell

net/ethtool/phy.c

@@ -164,7 +164,7 @@ int ethnl_phy_doit(struct sk_buff *skb, struct genl_info *info)
 		goto err_unlock_rtnl;
 
 	/* No PHY, return early */
-	if (!req_info.pdn->phy)
+	if (!req_info.pdn)
 		goto err_unlock_rtnl;
 
 	ret = ethnl_phy_reply_size(&req_info.base, info->extack);