[PATCH net] net: bridge: mdb: restart port group walk after deletion

[Fourie Zhang <littleddfu@gmail.com>]

br_mdb_flush_pgs() keeps a pointer-to-pointer cursor while walking
mp->ports. br_multicast_del_pg() can re-enter the same MDB entry through
br_multicast_sg_del_exclude_ports() and unlink other port groups. If the
cursor points into one of those groups, the next iteration dereferences a
stale cursor and can leave mp->ports pointing at freed memory.

A following RTM_GETMDB exposes the dangling pointer:

  BUG: KASAN: slab-use-after-free in br_mdb_dump
  Read of size 8
    br_mdb_dump
    rtnl_mdb_dump
    rtnl_dumpit
    netlink_dump

Reset the cursor to mp->ports after every deletion. The deletion removes at
least the selected group, so the restarted walk always makes progress.

Fixes: a6acb535afb2 ("bridge: mdb: Add MDB bulk deletion support")
Cc: stable@vger.kernel.org
Assisted-by: tencentos-corvus-ai:hy4-preview
Signed-off-by: Fourie Zhang <fouriezhang@tencent.com>
---
A reproducer for this issue is available if requested.

 net/bridge/br_mdb.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/bridge/br_mdb.c b/net/bridge/br_mdb.c
index e0c7020b1..a01bd280c 100644
--- a/net/bridge/br_mdb.c
+++ b/net/bridge/br_mdb.c
@@ -1523,6 +1523,8 @@ static void br_mdb_flush_pgs(struct net_bridge *br,
 		}
 
 		br_multicast_del_pg(mp, p, pp);
+		/* br_multicast_del_pg() can remove other groups from this list. */
+		pp = &mp->ports;
 	}
 }
 
-- 
2.43.7