[PATCH 6.12.y] batman-adv: bla: avoid NULL-ptr deref for claim via dropped interface

[Sven Eckelmann <sven@narfation.org>]

commit f80d3d98d2ff78d9e2fe5d68b1f45948c4f7bd24 upstream.

Without rtnl_lock held, a hardif might be retrieved as primary interface of
a meshif, but then (while operating on this interface) getting decoupled
from the mesh interface. In this case, the meshif still exists but the
pointer from the primary hardif to the meshif is set to NULL.

The mesh_iface must be checked first to be non-NULL before continuing to
send an ARP request using meshif.

Cc: stable@kernel.org
Fixes: 23721387c409 ("batman-adv: add basic bridge loop avoidance code")
Reported-by: Ido Schimmel <idosch@nvidia.com>
Reported-by: syzbot+9fdcc9f05a98a540b816@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9fdcc9f05a98a540b816
[ switch to old "mesh_iface" name "soft_iface" ]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
 net/batman-adv/bridge_loop_avoidance.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/batman-adv/bridge_loop_avoidance.c b/net/batman-adv/bridge_loop_avoidance.c
index 87f86c2bc4f22..15aeb07285e61 100644
--- a/net/batman-adv/bridge_loop_avoidance.c
+++ b/net/batman-adv/bridge_loop_avoidance.c
@@ -356,12 +356,14 @@ static void batadv_bla_send_claim(struct batadv_priv *bat_priv, const u8 *mac,
 	       sizeof(local_claim_dest));
 	local_claim_dest.type = claimtype;
 
-	soft_iface = primary_if->soft_iface;
+	soft_iface = READ_ONCE(primary_if->soft_iface);
+	if (!soft_iface)
+		goto out;
 
 	skb = arp_create(ARPOP_REPLY, ETH_P_ARP,
 			 /* IP DST: 0.0.0.0 */
 			 zeroip,
-			 primary_if->soft_iface,
+			 soft_iface,
 			 /* IP SRC: 0.0.0.0 */
 			 zeroip,
 			 /* Ethernet DST: Broadcast */
-- 
2.47.3