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

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 a963d710f367 ("netfilter: ctnetlink: Fix regression in CTA_STATUS
processing"), which makes me feel a little confusing.

Fixes: 76507f69c44e ("[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>

1 files changed, 9 insertions, 4 deletions

diff --git a/include/uapi/linux/netfilter/nf_conntrack_common.h b/include/uapi/linux/netfilter/nf_conntrack_common.h
index 6a8e33dd4ecb..38fc383139f0 100644
--- a/include/uapi/linux/netfilter/nf_conntrack_common.h
+++ b/include/uapi/linux/netfilter/nf_conntrack_common.h
@@ -82,10 +82,6 @@ enum ip_conntrack_status {
 	IPS_DYING_BIT = 9,
 	IPS_DYING = (1 << IPS_DYING_BIT),
 
-	/* Bits that cannot be altered from userland. */
-	IPS_UNCHANGEABLE_MASK = (IPS_NAT_DONE_MASK | IPS_NAT_MASK |
-				 IPS_EXPECTED | IPS_CONFIRMED | IPS_DYING),
-
 	/* Connection has fixed timeout. */
 	IPS_FIXED_TIMEOUT_BIT = 10,
 	IPS_FIXED_TIMEOUT = (1 << IPS_FIXED_TIMEOUT_BIT),
@@ -101,6 +97,15 @@ enum ip_conntrack_status {
 	/* Conntrack got a helper explicitly attached via CT target. */
 	IPS_HELPER_BIT = 13,
 	IPS_HELPER = (1 << IPS_HELPER_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 = 14,
 };
 
 /* Connection tracking event types */