[PATCH net v2] rds: tcp: unregister sysctl before tearing down listen socket

rds_tcp_exit_net() frees the per-netns RDS TCP listen socket via
rds_tcp_kill_sock() before unregistering the per-netns sysctl table.  Since
rds_tcp_skbuf_handler() derives the netns from 
rtn->rds_tcp_listen_sock->sk, a concurrent sysctl write can race with
netns teardown and dereference the freed socket/sk.

KASAN reports the race as:

  BUG: KASAN: slab-use-after-free in rds_tcp_skbuf_handler+0x2aa/0x2e0
  rds_tcp_skbuf_handler              net/rds/tcp.c:721
  proc_sys_call_handler              fs/proc/proc_sysctl.c
  vfs_write                          fs/read_write.c
  __x64_sys_pwrite64                 fs/read_write.c

Fix this by unregistering the RDS TCP sysctl table before calling
rds_tcp_kill_sock().  unregister_net_sysctl_table() prevents new sysctl
handlers from starting and waits for in-flight handlers to finish, so
the listen socket can then be released safely. The fix was tested
against the linked reproducer.

Fixes: 7f5611cbc487 ("rds: sysctl: rds_tcp_{rcv,snd}buf: avoid using current->nsproxy")
Reported-by: AutonomousCodeSecurity@microsoft.com
Link: https://lore.kernel.org/all/20260719203718.9680-1-blbllhy@gmail.com (local)
Reviewed-by: Allison Henderson <achender@kernel.org>
Signed-off-by: Cen Zhang (Microsoft) <redacted>
---
v2:
  - Add link to KASAN report and reproducer code.
  - Add Reviewed-by tags.

 net/rds/tcp.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/rds/tcp.c b/net/rds/tcp.c
index a1de114d5e2e..453d4077a85e 100644
--- a/net/rds/tcp.c
+++ b/net/rds/tcp.c

@@ -655,13 +655,13 @@ static void __net_exit rds_tcp_exit_net(struct net *net)
 {
 	struct rds_tcp_net *rtn = net_generic(net, rds_tcp_netid);
 
-	rds_tcp_kill_sock(net);
-
 	if (rtn->rds_tcp_sysctl)
 		unregister_net_sysctl_table(rtn->rds_tcp_sysctl);
 
 	if (net != &init_net)
 		kfree(rtn->ctl_table);
+
+	rds_tcp_kill_sock(net);
 }
 
 static struct pernet_operations rds_tcp_net_ops = {

-- 
2.53.0