[PATCH net 7/9] sctp: sysctl: udp_port: avoid using current->nsproxy

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

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, but that would
increase the size of this fix, while 'sctp.ctl_sock' still needs to be
retrieved from 'net' structure.

Fixes: 046c052b475e ("sctp: enable udp tunneling socks")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/67769ecb.050a0220.3a8527.003f.GAE@google.com [1]
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/sctp/sysctl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sctp/sysctl.c b/net/sctp/sysctl.c
index 9d29611621feaf0d2e8d7c923601ab374515563b..18fa4f44e8ec8c86f8415b1251ef8a2979c7f823 100644
--- a/net/sctp/sysctl.c
+++ b/net/sctp/sysctl.c
@@ -528,7 +528,7 @@ static int proc_sctp_do_auth(const struct ctl_table *ctl, int write,
 static int proc_sctp_do_udp_port(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.udp_port);
 	unsigned int min = *(unsigned int *)ctl->extra1;
 	unsigned int max = *(unsigned int *)ctl->extra2;
 	struct ctl_table tbl;

-- 
2.47.1