[PATCH net] rxrpc: Take write lock when publishing the initial RxGK key

[Chengfeng Ye <nicoyip.dev@gmail.com>]

rxgk_rekey() updates the transport-key ring under security_use_lock,
and rxgk_get_key() takes a reference under the corresponding read
lock.  The initial publication in rxgk_init_connection_security()
writes conn->rxgk.enctype and conn->rxgk.keys[] without the write
lock.

On a client connection, a second sendmsg can observe
RXRPC_CONN_CLIENT through a lockless load of conn->state, skip
rxrpc_init_client_conn_security(), and call rxgk_get_key() without
ever acquiring security_lock.  Because the initializer never took
the write lock, the reader's read lock provides neither exclusion
nor a matching acquire-release pair.

Concurrent RxGK key consumers were observed in a two-sender
workload.  KCSAN reported:

  BUG: KCSAN: data-race in rxgk_get_key / rxgk_secure_packet

  write to 0xffff8aef4023c318 of 8 bytes by task 1968 on cpu 0:
   rxgk_secure_packet+0x46c/0x820
   rxrpc_send_data+0x562/0x1a20
   rxrpc_do_sendmsg+0x976/0xa80
   rxrpc_sendmsg+0x20f/0x2a0

  read to 0xffff8aef4023c318 of 8 bytes by task 1969 on cpu 1:
   rxgk_get_key+0x209/0x5e0
   rxgk_alloc_txbuf+0xa4/0x2a0
   rxrpc_send_data+0x8e2/0x1a20
   rxrpc_do_sendmsg+0x976/0xa80
   rxrpc_sendmsg+0x20f/0x2a0

  value changed: 0x7fffffffffffffff -> 0x7fffffffffffffee

That report is on the key context's byte counter rather than the
initial publication, but it shows that lookup and secured transmit
already overlap on the same connection.

Take security_use_lock for writing while publishing the initial
enctype and transport key, matching the locking used when rekeying.

Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 net/rxrpc/rxgk.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/rxrpc/rxgk.c b/net/rxrpc/rxgk.c
index 77a67ace1d24..b49221f57f67 100644
--- a/net/rxrpc/rxgk.c
+++ b/net/rxrpc/rxgk.c
@@ -251,8 +251,10 @@ static int rxgk_init_connection_security(struct rxrpc_connection *conn,
 					 GFP_NOFS);
 	if (IS_ERR(gk))
 		return PTR_ERR(gk);
+	write_lock(&conn->security_use_lock);
 	conn->rxgk.enctype = gk->krb5->etype;
 	conn->rxgk.keys[gk->key_number & 3] = gk;
+	write_unlock(&conn->security_use_lock);
 
 	switch (conn->security_level) {
 	case RXRPC_SECURITY_PLAIN:
-- 
2.43.0