netfilter: ctnetlink: make it safer when updating ct->status

commit 53b56da8 upstream.

After converting to use rcu for conntrack hash, one CPU may update
the ct->status via ctnetlink, while another CPU may process the
packets and update the ct->status.

So the non-atomic operation "ct->status |= status;" via ctnetlink
becomes unsafe, and this may clear the IPS_DYING_BIT bit set by
another CPU unexpectedly. For example:
         CPU0                            CPU1
  ctnetlink_change_status        __nf_conntrack_find_get
      old = ct->status              nf_ct_gc_expired
          -                         nf_ct_kill
          -                      test_and_set_bit(IPS_DYING_BIT
      new = old | status;                 -
  ct->status = new; <-- oops, _DYING_ is cleared!

Now using a series of atomic bit operation to solve the above issue.

Also note, user shouldn't set IPS_TEMPLATE, IPS_SEQ_ADJUST directly,
so make these two bits be unchangable too.

If we set the IPS_TEMPLATE_BIT, ct will be freed by nf_ct_tmpl_free,
but actually it is alloced by nf_conntrack_alloc.
If we set the IPS_SEQ_ADJUST_BIT, this may cause the NULL pointer
deference, as the nfct_seqadj(ct) maybe NULL.

Last, add some comments to describe the logic change due to the
commit a963d710 ("netfilter: ctnetlink: Fix regression in CTA_STATUS
processing"), which makes me feel a little confusing.

Fixes: 76507f69 ("[NETFILTER]: nf_conntrack: use RCU for conntrack hash")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
[bwh: Backported to 3.2:
 - IPS_UNCHANGEABLE_MASK was not previously defined and ctnetlink_update_status()
   is not needed
 - enum ip_conntrack_status only assigns 13 bits
 - Adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

include/linux/netfilter/nf_conntrack_common.h

@@ -83,6 +83,15 @@ enum ip_conntrack_status {
 	/* Conntrack is a fake untracked entry */
 	IPS_UNTRACKED_BIT = 12,
 	IPS_UNTRACKED = (1 << IPS_UNTRACKED_BIT),
+
+	/* Be careful here, modifying these bits can make things messy,
+	 * so don't let users modify them directly.
+	 */
+	IPS_UNCHANGEABLE_MASK = (IPS_NAT_DONE_MASK | IPS_NAT_MASK |
+				 IPS_EXPECTED | IPS_CONFIRMED | IPS_DYING |
+				 IPS_SEQ_ADJUST | IPS_TEMPLATE),
+
+	__IPS_MAX_BIT = 13,
 };
 
 /* Connection tracking event types */

net/netfilter/nf_conntrack_netlink.c

@@ -1053,6 +1053,24 @@ ctnetlink_parse_nat_setup(struct nf_conn *ct,
 }
 #endif
 
+static void
+__ctnetlink_change_status(struct nf_conn *ct, unsigned long on,
+			  unsigned long off)
+{
+	unsigned int bit;
+
+	/* Ignore these unchangable bits */
+	on &= ~IPS_UNCHANGEABLE_MASK;
+	off &= ~IPS_UNCHANGEABLE_MASK;
+
+	for (bit = 0; bit < __IPS_MAX_BIT; bit++) {
+		if (on & (1 << bit))
+			set_bit(bit, &ct->status);
+		else if (off & (1 << bit))
+			clear_bit(bit, &ct->status);
+	}
+}
+
 static int
 ctnetlink_change_status(struct nf_conn *ct, const struct nlattr * const cda[])
 {
@@ -1072,10 +1090,7 @@ ctnetlink_change_status(struct nf_conn *ct, const struct nlattr * const cda[])
 		/* ASSURED bit can only be set */
 		return -EBUSY;
 
-	/* Be careful here, modifying NAT bits can screw up things,
-	 * so don't let users modify them directly if they don't pass
-	 * nf_nat_range. */
-	ct->status |= status & ~(IPS_NAT_DONE_MASK | IPS_NAT_MASK);
+	__ctnetlink_change_status(ct, status, 0);
 	return 0;
 }
 
@@ -1258,7 +1273,7 @@ ctnetlink_change_nat_seq_adj(struct nf_conn *ct,
 		if (ret < 0)
 			return ret;
 
-		ct->status |= IPS_SEQ_ADJUST;
+		set_bit(IPS_SEQ_ADJUST_BIT, &ct->status);
 	}
 
 	if (cda[CTA_NAT_SEQ_ADJ_REPLY]) {
@@ -1267,7 +1282,7 @@ ctnetlink_change_nat_seq_adj(struct nf_conn *ct,
 		if (ret < 0)
 			return ret;
 
-		ct->status |= IPS_SEQ_ADJUST;
+		set_bit(IPS_SEQ_ADJUST_BIT, &ct->status);
 	}
 
 	return 0;