[PATCH net-next] rtnetlink: use dev_isalive() in rtnl_getlink()

[Eric Dumazet <edumazet@google.com>]

rtnl_getlink() uses an RCU lookup to get the netdevice pointer.

When/If rtnl_lock() is used, we should check if the netdevice is not
being dismantled before potentially perform illegal actions.

Move dev_isalive() out of net/core/net-sysfs.c and make it available
in include/linux/netdevice.h

Return -ENODEV if rtnl_getlink() finds a device which is currently
being dismantled and RTNL is requested.

Fixes: e896e5c0734b ("rtnetlink: do not acquire RTNL in rtnl_getlink() with RTEXT_FILTER_NAME_ONLY")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
---
 include/linux/netdevice.h | 6 ++++++
 net/core/net-sysfs.c      | 6 ------
 net/core/rtnetlink.c      | 5 +++++
 3 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 74507c006490f180d2fac6594f6dcf2c86d53919..bfc847bc47fd957b58174dc4d2b82e2e8b461405 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -5660,6 +5660,12 @@ static inline const char *netdev_name(const struct net_device *dev)
 	return dev->name;
 }
 
+/* Caller holds RTNL, netdev->lock or RCU */
+static inline bool dev_isalive(const struct net_device *dev)
+{
+	return READ_ONCE(dev->reg_state) <= NETREG_REGISTERED;
+}
+
 static inline const char *netdev_reg_state(const struct net_device *dev)
 {
 	u8 reg_state = READ_ONCE(dev->reg_state);
diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 3318b5666e438fe5bab2bcfed2bb260c7b4d5a63..0e71c9ed41e81d85af33a4339f556a0c5d760243 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -37,12 +37,6 @@ static const char fmt_uint[] = "%u\n";
 static const char fmt_ulong[] = "%lu\n";
 static const char fmt_u64[] = "%llu\n";
 
-/* Caller holds RTNL, netdev->lock or RCU */
-static inline int dev_isalive(const struct net_device *dev)
-{
-	return READ_ONCE(dev->reg_state) <= NETREG_REGISTERED;
-}
-
 /* There is a possible ABBA deadlock between rtnl_lock and kernfs_node->active,
  * when unregistering a net device and accessing associated sysfs files. The
  * potential deadlock is as follow:
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 652dd008955a90691403de9a54d8693d64ea7799..9e90d02b73c2909409e12adfcec3940b49dcbaa1 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -4265,6 +4265,10 @@ static int rtnl_getlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 retry:
 	if (need_rtnl) {
 		rtnl_lock();
+		if (!dev_isalive(dev)) {
+			err = -ENODEV;
+			goto unlock;
+		}
 		/* Synchronize the carrier state so we don't report a state
 		 * that we're not actually going to honour immediately; if
 		 * the driver just did a carrier off->on transition, we can
@@ -4282,6 +4286,7 @@ static int rtnl_getlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 				       nlh->nlmsg_seq, 0, 0, ext_filter_mask,
 				       0, NULL, 0, netnsid, GFP_KERNEL);
 
+unlock:
 	if (need_rtnl)
 		rtnl_unlock();
 
-- 
2.54.0.929.g9b7fa37559-goog