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@xxxxxxxxx>
Link: https://lore.kernel.org/netfilter-devel/20250627142758.25664-1-fw@xxxxxxxxx/
Link: https://lore.kernel.org/netfilter-devel/4239da15-83ff-4ca4-939d-faef283471bb@xxxxxxxxx/
Fixes: 1397af5bfd7d ("netfilter: conntrack: remove the percpu dying list")
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
---
v2: prefer set_bit+memory barrier
amend commit message to mention this
add Link to v1 and to Razvans test module.
detach from original series, the first patches were selftest
patches that do not depend on this one.
include/net/netfilter/nf_conntrack.h | 15 +++++++++++++--
net/netfilter/nf_conntrack_core.c | 26 ++++++++++++++++++++------
2 files changed, 33 insertions(+), 8 deletions(-)
diff --git a/include/net/netfilter/nf_conntrack.h b/include/net/netfilter/nf_conntrack.h
index 3f02a45773e8..ca26274196b9 100644
--- a/include/net/netfilter/nf_conntrack.h
+++ b/include/net/netfilter/nf_conntrack.h
@@ -306,8 +306,19 @@ static inline bool nf_ct_is_expired(const struct nf_conn *ct)
/* 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)
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 7dbab9af25c9..427f37fee4fa 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -1124,6 +1124,12 @@ static int nf_ct_resolve_clash_harder(struct sk_buff *skb, u32 repl_idx)
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;
@@ -1261,8 +1267,6 @@ __nf_conntrack_confirm(struct sk_buff *skb)
* 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);
WARN_ON_ONCE(1);
@@ -1298,7 +1302,7 @@ __nf_conntrack_confirm(struct sk_buff *skb)
}
}
- /* 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;
@@ -1306,11 +1310,21 @@ __nf_conntrack_confirm(struct sk_buff *skb)
__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();
--
2.49.1
