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

net: bridge: fix vlan tunnel dst refcnt when egressing

The egress tunnel code uses dst_clone() and directly sets the result
which is wrong because the entry might have 0 refcnt or be already deleted,
causing number of problems. It also triggers the WARN_ON() in dst_hold()[1]
when a refcnt couldn’t be taken. Fix it by using dst_hold_safe() and
checking if a reference was actually taken before setting the dst.

[1] dmesg WARN_ON log and following refcnt errors
WARNING: CPU: 5 PID: 38 at include/net/dst.h:230 br_handle_egress_vlan_tunnel+0x10b/0x134 [bridge]
Modules linked in: 8021q garp mrp bridge stp llc bonding ipv6 virtio_net
CPU: 5 PID: 38 Comm: ksoftirqd/5 Kdump: loaded Tainted: G W 5.13.0-rc3+ #360
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-1.fc33 04/01/2014
RIP: 0010:br_handle_egress_vlan_tunnel+0x10b/0x134 [bridge]
Code: e8 85 bc 01 e1 45 84 f6 74 90 45 31 f6 85 db 48 c7 c7 a0 02 19 a0 41 0f 94 c6 31 c9 31 d2 44 89 f6 e8 64 bc 01 e1 85 db 75 02 <0f> 0b 31 c9 31 d2 44 89 f6 48 c7 c7 70 02 19 a0 e8 4b bc 01 e1 49
RSP: 0018:ffff8881003d39e8 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffffa01902a0
RBP: ffff8881040c6700 R08: 0000000000000000 R09: 0000000000000001
R10: 2ce93d0054fe0d00 R11: 54fe0d00000e0000 R12: ffff888109515000
R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000401
FS: 0000000000000000(0000) GS:ffff88822bf40000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f42ba70f030 CR3: 0000000109926000 CR4: 00000000000006e0
Call Trace:
br_handle_vlan+0xbc/0xca [bridge]
__br_forward+0x23/0x164 [bridge]
deliver_clone+0x41/0x48 [bridge]
br_handle_frame_finish+0x36f/0x3aa [bridge]
? skb_dst+0x2e/0x38 [bridge]
? br_handle_ingress_vlan_tunnel+0x3e/0x1c8 [bridge]
? br_handle_frame_finish+0x3aa/0x3aa [bridge]
br_handle_frame+0x2c3/0x377 [bridge]
? __skb_pull+0x33/0x51
? vlan_do_receive+0x4f/0x36a
? br_handle_frame_finish+0x3aa/0x3aa [bridge]
__netif_receive_skb_core+0x539/0x7c6
? __list_del_entry_valid+0x16e/0x1c2
__netif_receive_skb_list_core+0x6d/0xd6
netif_receive_skb_list_internal+0x1d9/0x1fa
gro_normal_list+0x22/0x3e
dev_gro_receive+0x55b/0x600
? detach_buf_split+0x58/0x140
napi_gro_receive+0x94/0x12e
virtnet_poll+0x15d/0x315 [virtio_net]
__napi_poll+0x2c/0x1c9
net_rx_action+0xe6/0x1fb
__do_softirq+0x115/0x2d8
run_ksoftirqd+0x18/0x20
smpboot_thread_fn+0x183/0x19c
? smpboot_unregister_percpu_thread+0x66/0x66
kthread+0x10a/0x10f
? kthread_mod_delayed_work+0xb6/0xb6
ret_from_fork+0x22/0x30
—–[ end trace 49f61b07f775fd2b ]—-
dst_release: dst:00000000c02d677a refcnt:-1
dst_release underflow