Re: [patch net-next v3] net: ethtool: fix unheld rtnl lock

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 27 Aug 2024 09:23:36 +0200 Maxime Chevallier wrote:
> On Mon, 26 Aug 2024 14:38:53 -0300
> Diogo Jahchan Koike <djahchankoike@gmail.com> wrote:
> 
> > ethnl_req_get_phydev should be called with rtnl lock held.
> > 
> > Reported-by: syzbot+ec369e6d58e210135f71@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=ec369e6d58e210135f71
> > Fixes: 31748765bed3 ("net: ethtool: pse-pd: Target the command to the requested PHY")
> > Signed-off-by: Diogo Jahchan Koike <djahchankoike@gmail.com>  
> 
> This looks good to me.
> 
> Even though RTNL is released between the .validate() and .set()
> calls, should the PHY disappear, the .set() callback handles that. 
> 
> Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

I know this isn't very well documented, but the point of .set_validate
is to perform checks before taking rtnl_lock (which may be quite
heavily contended), and potentially skip .set completely.
See 99132b6eb792 ("ethtool: netlink: handle SET intro/outro in the
common code"). Since we take rtnl lock and always return 1, this starts
to feel a bit cart before the horse.

How about we move the validation into set? (following code for
illustration only, please modify/test/review carefully and submit
as v4 if agreed on):

diff --git a/net/ethtool/pse-pd.c b/net/ethtool/pse-pd.c
index ff81aa749784..18759d8f85a5 100644
--- a/net/ethtool/pse-pd.c
+++ b/net/ethtool/pse-pd.c
@@ -217,13 +217,10 @@ const struct nla_policy ethnl_pse_set_policy[ETHTOOL_A_PSE_MAX + 1] = {
 };
 
 static int
-ethnl_set_pse_validate(struct ethnl_req_info *req_info, struct genl_info *info)
+ethnl_set_pse_validate(struct phy_device *phydev, struct genl_info *info)
 {
-	struct net_device *dev = req_info->dev;
 	struct nlattr **tb = info->attrs;
-	struct phy_device *phydev;
 
-	phydev = dev->phydev;
 	if (!phydev) {
 		NL_SET_ERR_MSG(info->extack, "No PHY is attached");
 		return -EOPNOTSUPP;
@@ -249,7 +246,7 @@ ethnl_set_pse_validate(struct ethnl_req_info *req_info, struct genl_info *info)
 		return -EOPNOTSUPP;
 	}
 
-	return 1;
+	return 0;
 }
 
 static int
@@ -258,10 +255,14 @@ ethnl_set_pse(struct ethnl_req_info *req_info, struct genl_info *info)
 	struct net_device *dev = req_info->dev;
 	struct nlattr **tb = info->attrs;
 	struct phy_device *phydev;
-	int ret = 0;
+	int ret;
 
 	phydev = dev->phydev;
 
+	ret = ethnl_set_pse_validate(phydev, info);
+	if (ret)
+		return ret;
+
 	if (tb[ETHTOOL_A_C33_PSE_AVAIL_PW_LIMIT]) {
 		unsigned int pw_limit;
 
@@ -307,7 +308,6 @@ const struct ethnl_request_ops ethnl_pse_request_ops = {
 	.fill_reply		= pse_fill_reply,
 	.cleanup_data		= pse_cleanup_data,
 
-	.set_validate		= ethnl_set_pse_validate,
 	.set			= ethnl_set_pse,
 	/* PSE has no notification */
 };