mptcp: sysctl: blackhole timeout: avoid using current->nsproxy

As mentioned in the previous commit, 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 'pernet' structure can be obtained from the table->data using
container_of().

Fixes: 27069e7 ("mptcp: disable active MPTCP in case of blackhole")
Link: https://lore.kernel.org/[email protected] [1]
Suggested-by: Al Viro <[email protected]>
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

Author: matttbe

net/mptcp/ctrl.c

@@ -160,7 +160,9 @@ static int proc_blackhole_detect_timeout(const struct ctl_table *table,
 					 int write, void *buffer, size_t *lenp,
 					 loff_t *ppos)
 {
-	struct mptcp_pernet *pernet = mptcp_get_pernet(current->nsproxy->net_ns);
+	struct mptcp_pernet *pernet = container_of(table->data,
+						   struct mptcp_pernet,
+						   blackhole_timeout);
 	int ret;
 
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);