[Bridge] Use-after-free in br_multicast_rcv

[Martin Weinelt]

Hello,

we've recently been experiencing memory leaks on our Linux-based routers,
at least as far back as v4.19.16.

After rebuilding with KASAN it found a use-after-free in 
br_multicast_rcv which I could reproduce on v5.2.0-rc6. 

Please find the KASAN report below, I'm not sure what else to provide so
feel free to ask.

Best,
  Martin


==================================================================
BUG: KASAN: use-after-free in br_multicast_rcv+0x480c/0x4ad0 [bridge]
Read of size 2 at addr ffff8880421302b4 by task ksoftirqd/1/16

CPU: 1 PID: 16 Comm: ksoftirqd/1 Tainted: G           OE     5.2.0-rc6+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
Call Trace:
 dump_stack+0x71/0xab
 print_address_description+0x6a/0x280
 ? br_multicast_rcv+0x480c/0x4ad0 [bridge]
 __kasan_report+0x152/0x1aa
 ? br_multicast_rcv+0x480c/0x4ad0 [bridge]
 ? br_multicast_rcv+0x480c/0x4ad0 [bridge]
 kasan_report+0xe/0x20
 br_multicast_rcv+0x480c/0x4ad0 [bridge]
 ? br_multicast_disable_port+0x150/0x150 [bridge]
 ? ktime_get_with_offset+0xb4/0x150
 ? __kasan_kmalloc.constprop.6+0xa6/0xf0
 ? __netif_receive_skb+0x1b0/0x1b0
 ? br_fdb_update+0x10e/0x6e0 [bridge]
 ? br_handle_frame_finish+0x3c6/0x11d0 [bridge]
 br_handle_frame_finish+0x3c6/0x11d0 [bridge]
 ? br_pass_frame_up+0x3a0/0x3a0 [bridge]
 ? virtnet_probe+0x1c80/0x1c80 [virtio_net]
 br_handle_frame+0x731/0xd90 [bridge]
 ? select_idle_sibling+0x25/0x7d0
 ? br_handle_frame_finish+0x11d0/0x11d0 [bridge]
 __netif_receive_skb_core+0xced/0x2d70
 ? virtqueue_get_buf_ctx+0x230/0x1130 [virtio_ring]
 ? do_xdp_generic+0x20/0x20
 ? virtqueue_napi_complete+0x39/0x70 [virtio_net]
 ? virtnet_poll+0x94d/0xc78 [virtio_net]
 ? receive_buf+0x5120/0x5120 [virtio_net]
 ? __netif_receive_skb_one_core+0x97/0x1d0
 __netif_receive_skb_one_core+0x97/0x1d0
 ? __netif_receive_skb_core+0x2d70/0x2d70
 ? _raw_write_trylock+0x100/0x100
 ? __queue_work+0x41e/0xbe0
 process_backlog+0x19c/0x650
 ? _raw_read_lock_irq+0x40/0x40
 net_rx_action+0x71e/0xbc0
 ? __switch_to_asm+0x40/0x70
 ? napi_complete_done+0x360/0x360
 ? __switch_to_asm+0x34/0x70
 ? __switch_to_asm+0x40/0x70
 ? __schedule+0x85e/0x14d0
 __do_softirq+0x1db/0x5f9
 ? takeover_tasklets+0x5f0/0x5f0
 run_ksoftirqd+0x26/0x40
 smpboot_thread_fn+0x443/0x680
 ? sort_range+0x20/0x20
 ? schedule+0x94/0x210
 ? __kthread_parkme+0x78/0xf0
 ? sort_range+0x20/0x20
 kthread+0x2ae/0x3a0
 ? kthread_create_worker_on_cpu+0xc0/0xc0
 ret_from_fork+0x35/0x40

The buggy address belongs to the page:
page:ffffea0001084c00 refcount:0 mapcount:-128 mapping:0000000000000000 index:0x0
flags: 0xffffc000000000()
raw: 00ffffc000000000 ffffea0000cfca08 ffffea0001098608 0000000000000000
raw: 0000000000000000 0000000000000003 00000000ffffff7f 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888042130180: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888042130200: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff888042130280: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                                     ^
 ffff888042130300: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888042130380: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================
Disabling lock debugging due to kernel taint