netfilter: nf_tables: don't queue packet path object notifications

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_tables: don't queue packet path object notifications

All file:line references below are against v7.2-rc4 (ac5b0e5651b1). The
trace was captured on 7.2.0-rc6-kasan72rc6 (075b74841bd0), where the same
lines apply.

nft_obj_notify() is exported and reached from the packet path. Its only
in-tree caller is nft_quota_obj_eval() (net/netfilter/nft_quota.c:68),
which notifies with GFP_ATOMIC while evaluating a rule for a transiting
packet, holding no mutex.

Since commit 67cc570edaa0 ("netfilter: nf_tables: coalesce multiple
notifications into one skbuff") that notification is no longer sent
immediately. __nft_obj_notify() queues it onto nft_net->notify_list via
nft_notify_enqueue() (net/netfilter/nf_tables_api.c:1211), which is a bare
list_add_tail(). notify_list has no lock of its own
(include/net/netfilter/nf_tables.h:1951), it is serialised by commit_mutex:
the six other enqueue sites all run inside a netlink transaction, and the
drain in nft_commit_notify() (net/netfilter/nf_tables_api.c:10746) does
list_del() + kfree_skb() from nf_tables_commit() with commit_mutex held.

Sending packets through a chain that references a depleted quota object
therefore races an unlocked list_add_tail() against list_del() +
kfree_skb() on another CPU. The WRITE_ONCE(prev->next, new) in __list_add()
then stores through an sk_buff that has already been freed:

BUG: KASAN: slab-use-after-free in __nft_obj_notify+0x2c5/0x2d0
Write of size 8 at addr ff110001047183c0 by task poc/76
CPU: 0 UID: 1000 PID: 76 Comm: poc Tainted: G W 7.2.0-rc6-kasan72rc6 #4
Call Trace:
<IRQ>
__nft_obj_notify (include/linux/list.h:164 include/linux/list.h:191
net/netfilter/nf_tables_api.c:1211
net/netfilter/nf_tables_api.c:8743)
nft_quota_obj_eval (net/netfilter/nft_quota.c:68)
nft_do_chain_inet
nf_hook_slow
__ip_local_out
ip_push_pending_frames
udp_send_skb
udp_sendmsg
__x64_sys_sendto

Allocated by task 77:
__alloc_skb (net/core/skbuff.c:704)
__nft_obj_notify (include/net/netlink.h:1055
net/netfilter/nf_tables_api.c:8731)
nft_quota_obj_eval (net/netfilter/nft_quota.c:68)
nft_do_chain

Freed by task 79:
nf_tables_commit (include/linux/skbuff.h:1332
net/netfilter/nf_tables_api.c:10759
net/netfilter/nf_tables_api.c:11185)
nfnetlink_rcv_batch (net/netfilter/nfnetlink.c:574)
netlink_unicast
netlink_sendmsg

The buggy address belongs to the cache skbuff_head_cache of size 232

Queueing from the packet path is wrong even leaving the race aside:
notify_list is only drained by nft_commit_notify() from nf_tables_commit()
(:11185), so a notification enqueued outside a transaction is not sent
until some later netlink batch commits, if one ever does.

The gfp argument that nft_obj_notify() still takes is a leftover of the
pre-67cc570edaa0 behaviour, where this path called nfnetlink_send()
directly. Restore that: split the message construction out into
nft_obj_notify_alloc() and let each caller decide what to do with the skb.
nft_obj_notify(), the exported one reached from the packet path, sends it
straight away; nf_tables_obj_notify(), which runs under commit_mutex, keeps
queueing it, so transaction notifications are still coalesced.