[PATCH net 4/9] sctp: sysctl: cookie_hmac_alg: 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.sctp_hmac_alg' is
used.

Fixes: 3c68198e7511 ("sctp: Make hmac algorithm selection for cookie generation dynamic")
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 e5a5af343c4c98de1afb27359c104f5030583ac4..9848d19630a4f760238a3a2abd3ec823f012d34a 100644
--- a/net/sctp/sysctl.c
+++ b/net/sctp/sysctl.c
@@ -387,7 +387,8 @@ static struct ctl_table sctp_net_table[] = {
 static int proc_sctp_do_hmac_alg(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.sctp_hmac_alg);
 	struct ctl_table tbl;
 	bool changed = false;
 	char *none = "none";

-- 
2.47.1