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2d72afb340
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
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|---|---|---|
| .. | ||
| ipset | ||
| ipvs | ||
| core.c | ||
| Kconfig | ||
| Makefile | ||
| nf_bpf_link.c | ||
| nf_conncount.c | ||
| nf_conntrack_acct.c | ||
| nf_conntrack_amanda.c | ||
| nf_conntrack_bpf.c | ||
| nf_conntrack_broadcast.c | ||
| nf_conntrack_core.c | ||
| nf_conntrack_ecache.c | ||
| nf_conntrack_expect.c | ||
| nf_conntrack_extend.c | ||
| nf_conntrack_ftp.c | ||
| nf_conntrack_h323_asn1.c | ||
| nf_conntrack_h323_main.c | ||
| nf_conntrack_h323_types.c | ||
| nf_conntrack_helper.c | ||
| nf_conntrack_irc.c | ||
| nf_conntrack_labels.c | ||
| nf_conntrack_netbios_ns.c | ||
| nf_conntrack_netlink.c | ||
| nf_conntrack_ovs.c | ||
| nf_conntrack_pptp.c | ||
| nf_conntrack_proto_dccp.c | ||
| nf_conntrack_proto_generic.c | ||
| nf_conntrack_proto_gre.c | ||
| nf_conntrack_proto_icmp.c | ||
| nf_conntrack_proto_icmpv6.c | ||
| nf_conntrack_proto_sctp.c | ||
| nf_conntrack_proto_tcp.c | ||
| nf_conntrack_proto_udp.c | ||
| nf_conntrack_proto.c | ||
| nf_conntrack_sane.c | ||
| nf_conntrack_seqadj.c | ||
| nf_conntrack_sip.c | ||
| nf_conntrack_snmp.c | ||
| nf_conntrack_standalone.c | ||
| nf_conntrack_tftp.c | ||
| nf_conntrack_timeout.c | ||
| nf_conntrack_timestamp.c | ||
| nf_dup_netdev.c | ||
| nf_flow_table_bpf.c | ||
| nf_flow_table_core.c | ||
| nf_flow_table_inet.c | ||
| nf_flow_table_ip.c | ||
| nf_flow_table_offload.c | ||
| nf_flow_table_procfs.c | ||
| nf_flow_table_xdp.c | ||
| nf_hooks_lwtunnel.c | ||
| nf_internals.h | ||
| nf_log_syslog.c | ||
| nf_log.c | ||
| nf_nat_amanda.c | ||
| nf_nat_bpf.c | ||
| nf_nat_core.c | ||
| nf_nat_ftp.c | ||
| nf_nat_helper.c | ||
| nf_nat_irc.c | ||
| nf_nat_masquerade.c | ||
| nf_nat_ovs.c | ||
| nf_nat_proto.c | ||
| nf_nat_redirect.c | ||
| nf_nat_sip.c | ||
| nf_nat_tftp.c | ||
| nf_queue.c | ||
| nf_sockopt.c | ||
| nf_synproxy_core.c | ||
| nf_tables_api.c | ||
| nf_tables_core.c | ||
| nf_tables_offload.c | ||
| nf_tables_trace.c | ||
| nfnetlink_acct.c | ||
| nfnetlink_cthelper.c | ||
| nfnetlink_cttimeout.c | ||
| nfnetlink_hook.c | ||
| nfnetlink_log.c | ||
| nfnetlink_osf.c | ||
| nfnetlink_queue.c | ||
| nfnetlink.c | ||
| nft_bitwise.c | ||
| nft_byteorder.c | ||
| nft_chain_filter.c | ||
| nft_chain_nat.c | ||
| nft_chain_route.c | ||
| nft_cmp.c | ||
| nft_compat.c | ||
| nft_connlimit.c | ||
| nft_counter.c | ||
| nft_ct_fast.c | ||
| nft_ct.c | ||
| nft_dup_netdev.c | ||
| nft_dynset.c | ||
| nft_exthdr.c | ||
| nft_fib_inet.c | ||
| nft_fib_netdev.c | ||
| nft_fib.c | ||
| nft_flow_offload.c | ||
| nft_fwd_netdev.c | ||
| nft_hash.c | ||
| nft_immediate.c | ||
| nft_inner.c | ||
| nft_last.c | ||
| nft_limit.c | ||
| nft_log.c | ||
| nft_lookup.c | ||
| nft_masq.c | ||
| nft_meta.c | ||
| nft_nat.c | ||
| nft_numgen.c | ||
| nft_objref.c | ||
| nft_osf.c | ||
| nft_payload.c | ||
| nft_queue.c | ||
| nft_quota.c | ||
| nft_range.c | ||
| nft_redir.c | ||
| nft_reject_inet.c | ||
| nft_reject_netdev.c | ||
| nft_reject.c | ||
| nft_rt.c | ||
| nft_set_bitmap.c | ||
| nft_set_hash.c | ||
| nft_set_pipapo_avx2.c | ||
| nft_set_pipapo_avx2.h | ||
| nft_set_pipapo.c | ||
| nft_set_pipapo.h | ||
| nft_set_rbtree.c | ||
| nft_socket.c | ||
| nft_synproxy.c | ||
| nft_tproxy.c | ||
| nft_tunnel.c | ||
| nft_xfrm.c | ||
| utils.c | ||
| x_tables.c | ||
| xt_addrtype.c | ||
| xt_AUDIT.c | ||
| xt_bpf.c | ||
| xt_cgroup.c | ||
| xt_CHECKSUM.c | ||
| xt_CLASSIFY.c | ||
| xt_cluster.c | ||
| xt_comment.c | ||
| xt_connbytes.c | ||
| xt_connlabel.c | ||
| xt_connlimit.c | ||
| xt_connmark.c | ||
| xt_CONNSECMARK.c | ||
| xt_conntrack.c | ||
| xt_cpu.c | ||
| xt_CT.c | ||
| xt_dccp.c | ||
| xt_devgroup.c | ||
| xt_dscp.c | ||
| xt_DSCP.c | ||
| xt_ecn.c | ||
| xt_esp.c | ||
| xt_hashlimit.c | ||
| xt_helper.c | ||
| xt_hl.c | ||
| xt_HL.c | ||
| xt_HMARK.c | ||
| xt_IDLETIMER.c | ||
| xt_ipcomp.c | ||
| xt_iprange.c | ||
| xt_ipvs.c | ||
| xt_l2tp.c | ||
| xt_LED.c | ||
| xt_length.c | ||
| xt_limit.c | ||
| xt_LOG.c | ||
| xt_mac.c | ||
| xt_mark.c | ||
| xt_MASQUERADE.c | ||
| xt_multiport.c | ||
| xt_nat.c | ||
| xt_NETMAP.c | ||
| xt_nfacct.c | ||
| xt_NFLOG.c | ||
| xt_NFQUEUE.c | ||
| xt_osf.c | ||
| xt_owner.c | ||
| xt_physdev.c | ||
| xt_pkttype.c | ||
| xt_policy.c | ||
| xt_quota.c | ||
| xt_rateest.c | ||
| xt_RATEEST.c | ||
| xt_realm.c | ||
| xt_recent.c | ||
| xt_REDIRECT.c | ||
| xt_repldata.h | ||
| xt_sctp.c | ||
| xt_SECMARK.c | ||
| xt_set.c | ||
| xt_socket.c | ||
| xt_state.c | ||
| xt_statistic.c | ||
| xt_string.c | ||
| xt_tcpmss.c | ||
| xt_TCPMSS.c | ||
| xt_TCPOPTSTRIP.c | ||
| xt_tcpudp.c | ||
| xt_TEE.c | ||
| xt_time.c | ||
| xt_TPROXY.c | ||
| xt_TRACE.c | ||
| xt_u32.c | ||