CVE-2021-47394

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

netfilter: nf_tables: unlink table before deleting it

syzbot reports following UAF:
BUG: KASAN: use-after-free in memcmp+0x18f/0x1c0 lib/string.c:955
nla_strcmp+0xf2/0x130 lib/nlattr.c:836
nft_table_lookup.part.0+0x1a2/0x460 net/netfilter/nf_tables_api.c:570
nft_table_lookup net/netfilter/nf_tables_api.c:4064 [inline]
nf_tables_getset+0x1b3/0x860 net/netfilter/nf_tables_api.c:4064
nfnetlink_rcv_msg+0x659/0x13f0 net/netfilter/nfnetlink.c:285
netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2504

Problem is that all get operations are lockless, so the commit_mutex
held by nft_rcv_nl_event() isn’t enough to stop a parallel GET request
from doing read-accesses to the table object even after synchronize_rcu().

To avoid this, unlink the table first and store the table objects in
on-stack scratch space.