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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 a963d710f3 ("netfilter: ctnetlink: Fix regression in CTA_STATUS
processing"), which makes me feel a little confusing.

Fixes: 76507f69c4 ("[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>
This commit is contained in:
Liping Zhang 2017-04-17 21:18:57 +08:00 committed by Pablo Neira Ayuso
parent 88be4c09d9
commit 53b56da83d
2 changed files with 33 additions and 13 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
include/uapi/linux/netfilter/nf_conntrack_common.h
View File

@ -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 */

33
net/netfilter/nf_conntrack_netlink.c
View File

@ -1419,6 +1419,24 @@ 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[])
{
@ -1438,10 +1456,7 @@ 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;
}
@ -1628,7 +1643,7 @@ ctnetlink_change_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_SEQ_ADJ_REPLY]) {
@ -1637,7 +1652,7 @@ ctnetlink_change_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;
@ -2289,10 +2304,10 @@ ctnetlink_update_status(struct nf_conn *ct, const struct nlattr * const cda[])
/* This check is less strict than ctnetlink_change_status()
* because callers often flip IPS_EXPECTED bits when sending
* an NFQA_CT attribute to the kernel. So ignore the
* unchangeable bits but do not error out.
* unchangeable bits but do not error out. Also user programs
* are allowed to clear the bits that they are allowed to change.
*/
ct->status = (status & ~IPS_UNCHANGEABLE_MASK) |
(ct->status & IPS_UNCHANGEABLE_MASK);
__ctnetlink_change_status(ct, status, ~status);
return 0;
}