rds: sysctl: rds_tcp_{rcv,snd}buf: avoid using current->nsproxy

As mentioned in a previous commit of this series, using the 'net'
structure via 'current' is not recommended for different reasons:

- Inconsistency: getting info from the reader's/writer's netns vs only
  from the opener's netns.

- current->nsproxy can be NULL in some cases, resulting in an 'Oops'
  (null-ptr-deref), e.g. when the current task is exiting, as spotted by
  syzbot [1] using acct(2).

The per-netns structure can be obtained from the table->data using
container_of(), then the 'net' one can be retrieved from the listen
socket (if available).

Fixes: c6a58ff ("RDS: TCP: Add sysctl tunables for sndbuf/rcvbuf on rds-tcp socket")
Link: https://lore.kernel.org/[email protected] [1]
Suggested-by: Al Viro <[email protected]>
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

net/rds/tcp.c

@@ -61,8 +61,10 @@ static atomic_t rds_tcp_unloading = ATOMIC_INIT(0);
 
 static struct kmem_cache *rds_tcp_conn_slab;
 
-static int rds_tcp_skbuf_handler(const struct ctl_table *ctl, int write,
-				 void *buffer, size_t *lenp, loff_t *fpos);
+static int rds_tcp_sndbuf_handler(const struct ctl_table *ctl, int write,
+				  void *buffer, size_t *lenp, loff_t *fpos);
+static int rds_tcp_rcvbuf_handler(const struct ctl_table *ctl, int write,
+				  void *buffer, size_t *lenp, loff_t *fpos);
 
 static int rds_tcp_min_sndbuf = SOCK_MIN_SNDBUF;
 static int rds_tcp_min_rcvbuf = SOCK_MIN_RCVBUF;
@@ -74,7 +76,7 @@ static struct ctl_table rds_tcp_sysctl_table[] = {
 		/* data is per-net pointer */
 		.maxlen         = sizeof(int),
 		.mode           = 0644,
-		.proc_handler   = rds_tcp_skbuf_handler,
+		.proc_handler   = rds_tcp_sndbuf_handler,
 		.extra1		= &rds_tcp_min_sndbuf,
 	},
 #define	RDS_TCP_RCVBUF	1
@@ -83,7 +85,7 @@ static struct ctl_table rds_tcp_sysctl_table[] = {
 		/* data is per-net pointer */
 		.maxlen         = sizeof(int),
 		.mode           = 0644,
-		.proc_handler   = rds_tcp_skbuf_handler,
+		.proc_handler   = rds_tcp_rcvbuf_handler,
 		.extra1		= &rds_tcp_min_rcvbuf,
 	},
 };
@@ -682,10 +684,10 @@ static void rds_tcp_sysctl_reset(struct net *net)
 	spin_unlock_irq(&rds_tcp_conn_lock);
 }
 
-static int rds_tcp_skbuf_handler(const struct ctl_table *ctl, int write,
+static int rds_tcp_skbuf_handler(struct rds_tcp_net *rtn,
+				 const struct ctl_table *ctl, int write,
 				 void *buffer, size_t *lenp, loff_t *fpos)
 {
-	struct net *net = current->nsproxy->net_ns;
 	int err;
 
 	err = proc_dointvec_minmax(ctl, write, buffer, lenp, fpos);
@@ -694,11 +696,34 @@ static int rds_tcp_skbuf_handler(const struct ctl_table *ctl, int write,
 			*(int *)(ctl->extra1));
 		return err;
 	}
-	if (write)
+
+	if (write && rtn->rds_tcp_listen_sock && rtn->rds_tcp_listen_sock->sk) {
+		struct net *net = sock_net(rtn->rds_tcp_listen_sock->sk);
+
 		rds_tcp_sysctl_reset(net);
+	}
+
 	return 0;
 }
 
+static int rds_tcp_sndbuf_handler(const struct ctl_table *ctl, int write,
+				  void *buffer, size_t *lenp, loff_t *fpos)
+{
+	struct rds_tcp_net *rtn = container_of(ctl->data, struct rds_tcp_net,
+					       sndbuf_size);
+
+	return rds_tcp_skbuf_handler(rtn, ctl, write, buffer, lenp, fpos);
+}
+
+static int rds_tcp_rcvbuf_handler(const struct ctl_table *ctl, int write,
+				  void *buffer, size_t *lenp, loff_t *fpos)
+{
+	struct rds_tcp_net *rtn = container_of(ctl->data, struct rds_tcp_net,
+					       rcvbuf_size);
+
+	return rds_tcp_skbuf_handler(rtn, ctl, write, buffer, lenp, fpos);
+}
+
 static void rds_tcp_exit(void)
 {
 	rds_tcp_set_unloading();