[PATCH net 8/9] sctp: sysctl: plpmtud_probe_interval: avoid using current->nsproxy

[Matthieu Baerts (NGI0)]

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 'net' structure can be obtained from the table->data using
container_of().

Note that table->data could also be used directly, as this is the only
member needed from the 'net' structure, but that would increase the size
of this fix, to use '*data' everywhere 'net->sctp.probe_interval' is
used.

Fixes: d1e462a7a5f3 ("sctp: add probe_interval in sysctl and sock/asoc/transport")
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/67769ecb.050a0220.3a8527.003f.GAE@xxxxxxxxxx [1]
Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
 net/sctp/sysctl.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c
index 18fa4f44e8ec8c86f8415b1251ef8a2979c7f823..8e1e97be4df79f3245e2bbbeb0a75841abc67f58 100644
--- a/net/sctp/sysctl.c
+++ b/net/sctp/sysctl.c
@@ -569,7 +569,8 @@ static int proc_sctp_do_udp_port(const struct ctl_table *ctl, int write,
 static int proc_sctp_do_probe_interval(const struct ctl_table *ctl, int write,
 				       void *buffer, size_t *lenp, loff_t *ppos)
 {
-	struct net *net = current->nsproxy->net_ns;
+	struct net *net = container_of(ctl->data, struct net,
+				       sctp.probe_interval);
 	struct ctl_table tbl;
 	int ret, new_value;
 

-- 
2.47.1