CVE-2021-47452

In the Linux kernel, the following vulnerability has been resolved:
netfilter: nf_tables: skip netdev events generated on netns removal syzbot
reported following (harmless) WARN: WARNING: CPU: 1 PID: 2648 at
net/netfilter/core.c:468 nft_netdev_unregister_hooks
net/netfilter/nf_tables_api.c:230 [inline] nf_tables_unregister_hook
include/net/netfilter/nf_tables.h:1090 [inline]
__nft_release_basechain+0x138/0x640 net/netfilter/nf_tables_api.c:9524
nft_netdev_event net/netfilter/nft_chain_filter.c:351 [inline]
nf_tables_netdev_event+0x521/0x8a0 net/netfilter/nft_chain_filter.c:382
reproducer: unshare -n bash -c ‘ip link add br0 type bridge; nft add table
netdev t ; \ nft add chain netdev t ingress { type filter hook ingress
device “br0” \ priority 0; policy drop; }’ Problem is that when netns
device exit hooks create the UNREGISTER event, the .pre_exit hook for
nf_tables core has already removed the base hook. Notifier attempts to do
this again. The need to do base hook unregister unconditionally was needed
in the past, because notifier was last stage where reg->dev dereference was
safe. Now that nf_tables does the hook removal in .pre_exit, this isn’t
needed anymore.