net: Fix race condition in store_rps_map

There is a race condition in store_rps_map that allows jump label
count in rps_needed to go below zero. This can happen when
concurrently attempting to set and a clear map.

Scenario:

1. rps_needed count is zero
2. New map is assigned by setting thread, but rps_needed count _not_ yet
   incremented (rps_needed count still zero)
2. Map is cleared by second thread, old_map set to that just assigned
3. Second thread performs static_key_slow_dec, rps_needed count now goes
   negative

Fix is to increment or decrement rps_needed under the spinlock.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 4 deletions

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 194c1d03b2b3..39ec6949c1e6 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -726,14 +726,17 @@ static ssize_t store_rps_map(struct netdev_rx_queue *queue,
         old_map = rcu_dereference_protected(queue->rps_map,
                                             lockdep_is_held(&rps_map_lock));
         rcu_assign_pointer(queue->rps_map, map);
-        spin_unlock(&rps_map_lock);
 
         if (map)
                 static_key_slow_inc(&rps_needed);
-        if (old_map) {
-                kfree_rcu(old_map, rcu);
+        if (old_map)
                 static_key_slow_dec(&rps_needed);
-        }
+
+        spin_unlock(&rps_map_lock);
+
+        if (old_map)
+                kfree_rcu(old_map, rcu);
+
         free_cpumask_var(mask);
         return len;
 }