Hi

When using Healer(https://github.com/SunHao-0/healer/tree/dev) to fuzz
the Linux kernel, I found a use-after-free vulnerability in
macvlan_broadcast.
Hope the report can help you locate the problem.

Details:
commit:   5695e5161 Linux 5.11
git tree:       upstream
kernel config and reproducing program can be found in the attachment.
report:
==================================================================
BUG: KASAN: use-after-free in macvlan_broadcast+0x595/0x6e0
Read of size 4 at addr ffff88806ae70801 by task syz-executor392/8448

CPU: 0 PID: 8448 Comm: syz-executor392 Not tainted 5.4.0 #14
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.13.0-1ubuntu1.1 04/01/2014
Call Trace:
 dump_stack+0xf3/0x16a
 print_address_description.constprop.0.cold+0xd3/0x372
 __kasan_report.cold+0x75/0x9b
 kasan_report+0x12/0x20
 macvlan_broadcast+0x595/0x6e0
 macvlan_start_xmit+0x40e/0x630
 dev_direct_xmit+0x3f6/0x5f0
 packet_sendmsg+0x233b/0x5a60
 sock_sendmsg+0xd7/0x130
 __sys_sendto+0x21e/0x330
 __x64_sys_sendto+0xe1/0x1b0
 do_syscall_64+0xbf/0x640
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x453d6d
Code: c3 e8 77 2c 00 00 0f 1f 80 00 00 00 00 f3 0f 1e fa 48 89 f8 48
89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d
01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff89ba8378 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000453d6d
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 0000000000000040 R08: 0000000020000280 R09: 0000000000000014
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
R13: 00007fff89ba83e8 R14: 00007fff89ba83f0 R15: 0000000000000000

Allocated by task 8448:
 save_stack+0x1b/0x80
 __kasan_kmalloc.constprop.0+0xc2/0xd0
 __kmalloc_node_track_caller+0x116/0x410
 __kmalloc_reserve.isra.0+0x35/0xd0
 __alloc_skb+0xf3/0x5c0
 rtmsg_fib+0x16f/0x11a0
 fib_table_insert+0x66e/0x1560
 fib_magic+0x406/0x570
 fib_add_ifaddr+0x39a/0x520
 fib_netdev_event+0x3d0/0x560
 notifier_call_chain+0xc0/0x230
 __dev_notify_flags+0x125/0x2d0
 dev_change_flags+0x104/0x160
 do_setlink+0x999/0x32a0
 __rtnl_newlink+0xac8/0x14c0
 rtnl_newlink+0x68/0xa0
 rtnetlink_rcv_msg+0x4a4/0xb70
 netlink_rcv_skb+0x15e/0x410
 netlink_unicast+0x4d4/0x690
 netlink_sendmsg+0x8ae/0xd00
 sock_sendmsg+0xd7/0x130
 __sys_sendto+0x21e/0x330
 __x64_sys_sendto+0xe1/0x1b0
 do_syscall_64+0xbf/0x640
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 8448:
 save_stack+0x1b/0x80
 __kasan_slab_free+0x126/0x170
 kfree+0xfa/0x460
 skb_free_head+0x8b/0xa0
 pskb_expand_head+0x2bd/0xe80
 netlink_trim+0x203/0x260
 netlink_broadcast_filtered+0x61/0xbd0
 nlmsg_notify+0x15b/0x1a0
 rtmsg_fib+0x2eb/0x11a0
 fib_table_insert+0x66e/0x1560
 fib_magic+0x406/0x570
 fib_add_ifaddr+0x39a/0x520
 fib_netdev_event+0x3d0/0x560
 notifier_call_chain+0xc0/0x230
 __dev_notify_flags+0x125/0x2d0
 dev_change_flags+0x104/0x160
 do_setlink+0x999/0x32a0
 __rtnl_newlink+0xac8/0x14c0
 rtnl_newlink+0x68/0xa0
 rtnetlink_rcv_msg+0x4a4/0xb70
 netlink_rcv_skb+0x15e/0x410
 netlink_unicast+0x4d4/0x690
 netlink_sendmsg+0x8ae/0xd00
 sock_sendmsg+0xd7/0x130
 __sys_sendto+0x21e/0x330
 __x64_sys_sendto+0xe1/0x1b0
 do_syscall_64+0xbf/0x640
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff88806ae70800
 which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 1 bytes inside of
 1024-byte region [ffff88806ae70800, ffff88806ae70c00)
The buggy address belongs to the page:
page:ffffea0001ab9c00 refcount:1 mapcount:0 mapping:ffff88806b802280
index:0x0 compound_mapcount: 0
raw: 00fff00000010200 dead000000000100 dead000000000122 ffff88806b802280
raw: 0000000000000000 0000000000080008 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88806ae70700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff88806ae70780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff88806ae70800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff88806ae70880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88806ae70900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================