sctp: sysctl: cookie_hmac_alg: avoid using current->nsproxy

JIRA: https://issues.redhat.com/browse/RHEL-84590
Tested: compile only

commit ea62dd1
Author: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Date:   Wed Jan 8 16:34:32 2025 +0100

    sctp: sysctl: cookie_hmac_alg: avoid using current->nsproxy

    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: 3c68198 ("sctp: Make hmac algorithm selection for cookie generation dynamic")
    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>
    Link: https://patch.msgid.link/20250108-net-sysctl-current-nsproxy-v1-4-5df34b2083e8@kernel.org
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Xin Long <lxin@redhat.com>

Author: Xin Long

net/sctp/sysctl.c

@@ -391,7 +391,8 @@ static struct ctl_table sctp_net_table[] = {
 static int proc_sctp_do_hmac_alg(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";