rds_tcp_laddr_check() looks up a scoped IPv6 interface with
dev_get_by_index_rcu(), drops the RCU read-side lock, and only then
passes the bare struct net_device * into ipv6_chk_addr().

dev_get_by_index_rcu() only keeps the device alive within the same RCU
read-side section. After rcu_read_unlock(), a concurrent RTM_DELLINK can
free the net_device; ipv6_chk_addr() then dereferences the stale pointer
in __ipv6_chk_addr_and_flags() (e.g. l3mdev_master_dev_rcu(dev)), reading
freed memory.

Take a reference with dev_hold() before dropping the RCU lock and release
it with dev_put() after ipv6_chk_addr(), so the device cannot be freed
while in use. dev_put(NULL) is a no-op, so the scope_id == 0 path is
unaffected.

  BUG: KASAN: slab-use-after-free in __ipv6_chk_addr_and_flags (... net/ipv6/addrconf.c:1998)
  Read of size 8 at addr ffff8880106ec000 by task exploit/153
  Call Trace:
   ...
   kasan_report (mm/kasan/report.c:595)
   __ipv6_chk_addr_and_flags (... net/ipv6/addrconf.c:1998)
   ipv6_chk_addr (net/ipv6/addrconf.c:2031 net/ipv6/addrconf.c:1972)
   rds_tcp_laddr_check (net/rds/tcp.c:370)
   rds_bind (net/rds/bind.c:248)
   __sys_bind (net/socket.c:1920)
   __x64_sys_bind (net/socket.c:1956)
   do_syscall_64 (arch/x86/entry/syscall_64.c:63)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

Fixes: eee2fa6ab322 ("rds: Changing IP address internal representation to struct in6_addr")
Reported-by: Weiming Shi <redacted>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei <redacted>
---
 net/rds/tcp.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index a1de114d5e2e..204dcdc33c27 100644
--- a/net/rds/tcp.c
+++ b/net/rds/tcp.c

@@ -363,12 +363,16 @@ int rds_tcp_laddr_check(struct net *net, const struct in6_addr *addr,
 			rcu_read_unlock();
 			return -EADDRNOTAVAIL;
 		}
+		dev_hold(dev);
 		rcu_read_unlock();
 	}
 #if IS_ENABLED(CONFIG_IPV6)
 	ret = ipv6_chk_addr(net, addr, dev, 0);
+	dev_put(dev);
 	if (ret)
 		return 0;
+#else
+	dev_put(dev);
 #endif
 	return -EADDRNOTAVAIL;
 }

-- 
2.43.0