A crash in conntrack was reported while trying to unlink the conntrack
entry from the hash bucket list:
[exception RIP: __nf_ct_delete_from_lists+172]
[..]
#7 [
ff539b5a2b043aa0] nf_ct_delete at
ffffffffc124d421 [nf_conntrack]
#8 [
ff539b5a2b043ad0] nf_ct_gc_expired at
ffffffffc124d999 [nf_conntrack]
#9 [
ff539b5a2b043ae0] __nf_conntrack_find_get at
ffffffffc124efbc [nf_conntrack]
[..]
The nf_conn struct is marked as allocated from slab but appears to be in
a partially initialised state:
ct hlist pointer is garbage; looks like the ct hash value
(hence crash).
ct->status is equal to IPS_CONFIRMED|IPS_DYING, which is expected
ct->timeout is 30000 (=30s), which is unexpected.
Everything else looks like normal udp conntrack entry. If we ignore
ct->status and pretend its 0, the entry matches those that are newly
allocated but not yet inserted into the hash:
- ct hlist pointers are overloaded and store/cache the raw tuple hash
- ct->timeout matches the relative time expected for a new udp flow
rather than the absolute 'jiffies' value.
If it were not for the presence of IPS_CONFIRMED,
__nf_conntrack_find_get() would have skipped the entry.
Theory is that we did hit following race:
cpu x cpu y cpu z
found entry E found entry E
E is expired <preemption>
nf_ct_delete()
return E to rcu slab
init_conntrack
E is re-inited,
ct->status set to 0
reply tuplehash hnnode.pprev
stores hash value.
cpu y found E right before it was deleted on cpu x.
E is now re-inited on cpu z. cpu y was preempted before
checking for expiry and/or confirm bit.
->refcnt set to 1
E now owned by skb
->timeout set to 30000
If cpu y were to resume now, it would observe E as
expired but would skip E due to missing CONFIRMED bit.
nf_conntrack_confirm gets called
sets: ct->status |= CONFIRMED
This is wrong: E is not yet added
to hashtable.
cpu y resumes, it observes E as expired but CONFIRMED:
<resumes>
nf_ct_expired()
-> yes (ct->timeout is 30s)
confirmed bit set.
cpu y will try to delete E from the hashtable:
nf_ct_delete() -> set DYING bit
__nf_ct_delete_from_lists
Even this scenario doesn't guarantee a crash:
cpu z still holds the table bucket lock(s) so y blocks:
wait for spinlock held by z
CONFIRMED is set but there is no
guarantee ct will be added to hash:
"chaintoolong" or "clash resolution"
logic both skip the insert step.
reply hnnode.pprev still stores the
hash value.
unlocks spinlock
return NF_DROP
<unblocks, then
crashes on hlist_nulls_del_rcu pprev>
In case CPU z does insert the entry into the hashtable, cpu y will unlink
E again right away but no crash occurs.
Without 'cpu y' race, 'garbage' hlist is of no consequence:
ct refcnt remains at 1, eventually skb will be free'd and E gets
destroyed via: nf_conntrack_put -> nf_conntrack_destroy -> nf_ct_destroy.
To resolve this, move the IPS_CONFIRMED assignment after the table
insertion but before the unlock.
Pablo points out that the confirm-bit-store could be reordered to happen
before hlist add resp. the timeout fixup, so switch to set_bit and
before_atomic memory barrier to prevent this.
It doesn't matter if other CPUs can observe a newly inserted entry right
before the CONFIRMED bit was set:
Such event cannot be distinguished from above "E is the old incarnation"
case: the entry will be skipped.
Also change nf_ct_should_gc() to first check the confirmed bit.
The gc sequence is:
1. Check if entry has expired, if not skip to next entry
2. Obtain a reference to the expired entry.
3. Call nf_ct_should_gc() to double-check step 1.
nf_ct_should_gc() is thus called only for entries that already failed an
expiry check. After this patch, once the confirmed bit check passes
ct->timeout has been altered to reflect the absolute 'best before' date
instead of a relative time. Step 3 will therefore not remove the entry.
Without this change to nf_ct_should_gc() we could still get this sequence:
1. Check if entry has expired.
2. Obtain a reference.
3. Call nf_ct_should_gc() to double-check step 1:
4 - entry is still observed as expired
5 - meanwhile, ct->timeout is corrected to absolute value on other CPU
and confirm bit gets set
6 - confirm bit is seen
7 - valid entry is removed again
First do check 6), then 4) so the gc expiry check always picks up either
confirmed bit unset (entry gets skipped) or expiry re-check failure for
re-inited conntrack objects.
This change cannot be backported to releases before 5.19. Without
commit
8a75a2c17410 ("netfilter: conntrack: remove unconfirmed list")
|= IPS_CONFIRMED line cannot be moved without further changes.
Cc: Razvan Cojocaru <rzvncj@gmail.com>
Link: https://lore.kernel.org/netfilter-devel/20250627142758.25664-1-fw@strlen.de/
Link: https://lore.kernel.org/netfilter-devel/4239da15-83ff-4ca4-939d-faef283471bb@gmail.com/
Fixes: 1397af5bfd7d ("netfilter: conntrack: remove the percpu dying list")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
/* use after obtaining a reference count */
static inline bool nf_ct_should_gc(const struct nf_conn *ct)
{
- return nf_ct_is_expired(ct) && nf_ct_is_confirmed(ct) &&
- !nf_ct_is_dying(ct);
+ if (!nf_ct_is_confirmed(ct))
+ return false;
+
+ /* load ct->timeout after is_confirmed() test.
+ * Pairs with __nf_conntrack_confirm() which:
+ * 1. Increases ct->timeout value
+ * 2. Inserts ct into rcu hlist
+ * 3. Sets the confirmed bit
+ * 4. Unlocks the hlist lock
+ */
+ smp_acquire__after_ctrl_dep();
+
+ return nf_ct_is_expired(ct) && !nf_ct_is_dying(ct);
}
#define NF_CT_DAY (86400 * HZ)
hlist_nulls_add_head_rcu(&loser_ct->tuplehash[IP_CT_DIR_REPLY].hnnode,
&nf_conntrack_hash[repl_idx]);
+ /* confirmed bit must be set after hlist add, not before:
+ * loser_ct can still be visible to other cpu due to
+ * SLAB_TYPESAFE_BY_RCU.
+ */
+ smp_mb__before_atomic();
+ set_bit(IPS_CONFIRMED_BIT, &loser_ct->status);
NF_CT_STAT_INC(net, clash_resolve);
return NF_ACCEPT;
* user context, else we insert an already 'dead' hash, blocking
* further use of that particular connection -JM.
*/
- ct->status |= IPS_CONFIRMED;
-
if (unlikely(nf_ct_is_dying(ct))) {
NF_CT_STAT_INC(net, insert_failed);
goto dying;
}
}
- /* Timer relative to confirmation time, not original
+ /* Timeout is relative to confirmation time, not original
setting time, otherwise we'd get timer wrap in
weird delay cases. */
ct->timeout += nfct_time_stamp;
__nf_conntrack_insert_prepare(ct);
/* Since the lookup is lockless, hash insertion must be done after
- * starting the timer and setting the CONFIRMED bit. The RCU barriers
- * guarantee that no other CPU can find the conntrack before the above
- * stores are visible.
+ * setting ct->timeout. The RCU barriers guarantee that no other CPU
+ * can find the conntrack before the above stores are visible.
*/
__nf_conntrack_hash_insert(ct, hash, reply_hash);
+
+ /* IPS_CONFIRMED unset means 'ct not (yet) in hash', conntrack lookups
+ * skip entries that lack this bit. This happens when a CPU is looking
+ * at a stale entry that is being recycled due to SLAB_TYPESAFE_BY_RCU
+ * or when another CPU encounters this entry right after the insertion
+ * but before the set-confirm-bit below. This bit must not be set until
+ * after __nf_conntrack_hash_insert().
+ */
+ smp_mb__before_atomic();
+ set_bit(IPS_CONFIRMED_BIT, &ct->status);
+
nf_conntrack_double_unlock(hash, reply_hash);
local_bh_enable();
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