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: 27069e7cb3d1 ("mptcp: disable active MPTCP in case of blackhole")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/67769ecb.050a0220.3a8527.003f.GAE@google.com (local) [1]
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/ctrl.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
index 81c30aa02196d69c55799e5963f6591e416c8831..b0dd008e2114bce65ee3906bbdc19a5a4316cefa 100644
--- a/net/mptcp/ctrl.c
+++ b/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);

-- 
2.47.1