netfilter: ctnetlink: zero expect NAT fields when CTA_EXPECT_NAT absent

ctnetlink_alloc_expect() allocates expectations from a non-zeroing
slab cache via nf_ct_expect_alloc().  When CTA_EXPECT_NAT is not
present in the netlink message, saved_addr and saved_proto are
never initialized.  Stale data from a previous slab occupant can
then be dumped to userspace by ctnetlink_exp_dump_expect(), which
checks these fields to decide whether to emit CTA_EXPECT_NAT.

The safe sibling nf_ct_expect_init(), used by the packet path,
explicitly zeroes these fields.

Zero saved_addr, saved_proto and dir in the else branch, guarded
by IS_ENABLED(CONFIG_NF_NAT) since these fields only exist when
NAT is enabled.

Confirmed by priming the expect slab with NAT-bearing expectations,
freeing them, creating a new expectation without CTA_EXPECT_NAT,
and observing that the ctnetlink dump emits a spurious
CTA_EXPECT_NAT containing stale data from the prior allocation.

Fixes: 076a0ca ("netfilter: ctnetlink: add NAT support for expectations")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Qi Tang <tpluszz77@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: Qi Tang

net/netfilter/nf_conntrack_netlink.c

@@ -3588,6 +3588,12 @@ ctnetlink_alloc_expect(const struct nlattr * const cda[], struct nf_conn *ct,
 						 exp, nf_ct_l3num(ct));
 		if (err < 0)
 			goto err_out;
+#if IS_ENABLED(CONFIG_NF_NAT)
+	} else {
+		memset(&exp->saved_addr, 0, sizeof(exp->saved_addr));
+		memset(&exp->saved_proto, 0, sizeof(exp->saved_proto));
+		exp->dir = 0;
+#endif
 	}
 	return exp;
 err_out: