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.

Keep the RCU read-side lock held across the ipv6_chk_addr() call instead
of dropping it right after the lookup, so the device cannot be freed
while it is in use.

  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 | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index 955d92277d5a..30cfb0087f2c 100644
--- a/net/rds/tcp.c
+++ b/net/rds/tcp.c

@@ -355,23 +355,25 @@ int rds_tcp_laddr_check(struct net *net, const struct in6_addr *addr,
 	/* If the scope_id is specified, check only those addresses
 	 * hosted on the specified interface.
 	 */
+	rcu_read_lock();
 	if (scope_id != 0) {
-		rcu_read_lock();
 		dev = dev_get_by_index_rcu(net, scope_id);
 		/* scope_id is not valid... */
 		if (!dev) {
 			rcu_read_unlock();
 			return -EADDRNOTAVAIL;
 		}
-		rcu_read_unlock();
 	}
 #if IS_ENABLED(CONFIG_IPV6)
 	if (ipv6_mod_enabled()) {
 		ret = ipv6_chk_addr(net, addr, dev, 0);
-		if (ret)
+		if (ret) {
+			rcu_read_unlock();
 			return 0;
+		}
 	}
 #endif
+	rcu_read_unlock();
 	return -EADDRNOTAVAIL;
 }
 

-- 
2.43.0