[PATCH net] bonding: alb: re-check primary_is_promisc under RTNL in bond_alb_monitor

["Xiang Mei (Microsoft)" <xmei5@asu.edu>]

bond_alb_monitor() reads primary_is_promisc under RCU, then drops RCU and
takes RTNL via rtnl_trylock() before undoing the promiscuity it set on the
active slave. In that window the active slave can change under RTNL
(RTM_DELLINK -> __bond_release_one() -> bond_alb_handle_active_change()),
which already drops the promiscuity and clears primary_is_promisc. The
monitor still acts on the stale decision: if the slave was removed with no
failover, curr_active_slave is now NULL and the deref faults; if it failed
over, the stale dev_set_promiscuity(-1) underflows the new slave's
promiscuity counter and pins it in IFF_PROMISC.

  Oops: general protection fault, probably for non-canonical address ...
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  Workqueue: b42 bond_alb_monitor
  RIP: 0010:bond_alb_monitor (drivers/net/bonding/bond_alb.c:1600)
   process_one_work (kernel/workqueue.c:3322)
   worker_thread (kernel/workqueue.c:3486)
   kthread (kernel/kthread.c:436)
   ret_from_fork (arch/x86/kernel/process.c:158)
  Kernel panic - not syncing: Fatal exception

Re-check primary_is_promisc (and curr_active_slave) after taking RTNL so
the monitor only undoes an increment it still owns. The other bonding
monitors already re-read state under RTNL in their commit phase
(bond_miimon_commit/bond_ab_arp_commit); bond_alb_monitor() was the only
one acting on the pre-trylock decision.

Fixes: d0e81b7e2246 ("bonding: Acquire correct locks in alb for promisc change")
Reported-by: AutonomousCodeSecurity@microsoft.com
Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
---
 drivers/net/bonding/bond_alb.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/net/bonding/bond_alb.c b/drivers/net/bonding/bond_alb.c
index 2d37b07c8215..70458c5b23cc 100644
--- a/drivers/net/bonding/bond_alb.c
+++ b/drivers/net/bonding/bond_alb.c
@@ -1535,7 +1535,7 @@ void bond_alb_monitor(struct work_struct *work)
 					    alb_work.work);
 	struct alb_bond_info *bond_info = &(BOND_ALB_INFO(bond));
 	struct list_head *iter;
-	struct slave *slave;
+	struct slave *slave, *curr;
 
 	if (!bond_has_slaves(bond)) {
 		atomic_set(&bond_info->tx_rebalance_counter, 0);
@@ -1597,9 +1597,11 @@ void bond_alb_monitor(struct work_struct *work)
 			 * because a slave was disabled then
 			 * it can now leave promiscuous mode.
 			 */
-			dev_set_promiscuity(rtnl_dereference(bond->curr_active_slave)->dev,
-					    -1);
-			bond_info->primary_is_promisc = 0;
+			curr = rtnl_dereference(bond->curr_active_slave);
+			if (bond_info->primary_is_promisc && curr) {
+				dev_set_promiscuity(curr->dev, -1);
+				bond_info->primary_is_promisc = 0;
+			}
 
 			rtnl_unlock();
 			rcu_read_lock();
-- 
2.43.0