I've run into a bug where dev_hold() was being called after
netdev_wait_allrefs(). But at that point, the device is already going
away, and dev_hold() can't stop that anymore.

To make such problems easier to diagnose in the future:

 - For CONFIG_PCPU_DEV_REFCNT builds: Recheck in free_netdev() whether
   the net refcount has been elevated. If this is detected, WARN() and
   leak the object (to prevent worse consequences from a
   use-after-free).
 - For builds without CONFIG_PCPU_DEV_REFCNT: Set the refcount to zero.
   This signals to the generic refcount infrastructure that any attempt
   to increment the refcount later is a bug.

Signed-off-by: Jann Horn <jannh@google.com>
---
 net/core/dev.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 1baab07820f6..f7916c0d226d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -9949,8 +9949,18 @@ void netdev_run_todo(void)

                netdev_wait_allrefs(dev);

+               /* Drop the netdev refcount (which should be 1 at this point)
+                * to zero. If we're using the generic refcount code, this will
+                * tell it that any dev_hold() after this point is a bug.
+                */
+#ifdef CONFIG_PCPU_DEV_REFCNT
+               this_cpu_dec(*dev->pcpu_refcnt);
+               BUG_ON(netdev_refcnt_read(dev) != 0);
+#else
+               BUG_ON(!refcount_dec_and_test(&dev->dev_refcnt));
+#endif
+
                /* paranoia */
-               BUG_ON(netdev_refcnt_read(dev) != 1);
                BUG_ON(!list_empty(&dev->ptype_all));
                BUG_ON(!list_empty(&dev->ptype_specific));
                WARN_ON(rcu_access_pointer(dev->ip_ptr));

@@ -10293,6 +10303,12 @@ void free_netdev(struct net_device *dev)
        free_percpu(dev->xdp_bulkq);
        dev->xdp_bulkq = NULL;

+       /* Recheck in case someone called dev_hold() between
+        * netdev_wait_allrefs() and here.
+        */