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BUG: KASAN: use-after-free in soft_cursor+0x62c/0xdb0 drivers/video/fbdev/core/softcursor.c:61
Read of size 1 at addr ffff88809e2d3810 by task kworker/1:137/3218

CPU: 1 PID: 3218 Comm: kworker/1:137 Not tainted 5.6.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events_power_efficient fb_flashcursor
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1e9/0x30e lib/dump_stack.c:118
 print_address_description+0x74/0x5c0 mm/kasan/report.c:374
 __kasan_report+0x14b/0x1c0 mm/kasan/report.c:506
 kasan_report+0x25/0x50 mm/kasan/common.c:641
 soft_cursor+0x62c/0xdb0 drivers/video/fbdev/core/softcursor.c:61
 bit_cursor+0x178a/0x2180 drivers/video/fbdev/core/bitblit.c:386
 fb_flashcursor+0x73e/0xa10 drivers/video/fbdev/core/fbcon.c:417
 process_one_work+0x76e/0xfd0 kernel/workqueue.c:2266
 worker_thread+0xa7f/0x1450 kernel/workqueue.c:2412
 kthread+0x317/0x340 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Allocated by task 10920:
 save_stack mm/kasan/common.c:72 [inline]
 set_track mm/kasan/common.c:80 [inline]
 __kasan_kmalloc+0x118/0x1c0 mm/kasan/common.c:515
 __kmalloc_reserve net/core/skbuff.c:142 [inline]
 __alloc_skb+0xde/0x4f0 net/core/skbuff.c:210
 alloc_skb include/linux/skbuff.h:1081 [inline]
 alloc_skb_with_frags+0xb6/0x590 net/core/skbuff.c:5764
 sock_alloc_send_pskb+0x7a0/0x890 net/core/sock.c:2245
 __ip_append_data+0x20a2/0x33b0 net/ipv4/ip_output.c:1089
 ip_make_skb+0x1a5/0x440 net/ipv4/ip_output.c:1626
 udp_sendmsg+0x218f/0x2fb0 net/ipv4/udp.c:1173
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 ____sys_sendmsg+0x4f9/0x7c0 net/socket.c:2343
 ___sys_sendmsg net/socket.c:2397 [inline]
 __sys_sendmmsg+0x2a9/0x5b0 net/socket.c:2487
 __do_sys_sendmmsg net/socket.c:2516 [inline]
 __se_sys_sendmmsg net/socket.c:2513 [inline]
 __x64_sys_sendmmsg+0x9c/0xb0 net/socket.c:2513
 do_syscall_64+0xf3/0x1b0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 11201:
 save_stack mm/kasan/common.c:72 [inline]
 set_track mm/kasan/common.c:80 [inline]
 kasan_set_free_info mm/kasan/common.c:337 [inline]
 __kasan_slab_free+0x12e/0x1e0 mm/kasan/common.c:476
 __cache_free mm/slab.c:3426 [inline]
 kfree+0x10a/0x220 mm/slab.c:3757
 skb_release_all net/core/skbuff.c:664 [inline]
 __kfree_skb+0x56/0x1c0 net/core/skbuff.c:678
 tun_do_read+0xc9f/0x1450 drivers/net/tun.c:2236
 tun_chr_read_iter+0xc6/0x190 drivers/net/tun.c:2251
 call_read_iter include/linux/fs.h:1895 [inline]
 new_sync_read fs/read_write.c:414 [inline]
 __vfs_read+0x546/0x6f0 fs/read_write.c:427
 vfs_read+0x1c0/0x3f0 fs/read_write.c:461
 ksys_read+0x115/0x220 fs/read_write.c:587
 do_syscall_64+0xf3/0x1b0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff88809e2d3800
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 16 bytes inside of
 512-byte region [ffff88809e2d3800, ffff88809e2d3a00)
The buggy address belongs to the page:
page:ffffea000278b4c0 refcount:1 mapcount:0 mapping:ffff8880aa400a80 index:0xffff88809e2d3c00
flags: 0xfffe0000000200(slab)
raw: 00fffe0000000200 ffffea0002841f08 ffffea00014a8248 ffff8880aa400a80
raw: ffff88809e2d3c00 ffff88809e2d3000 0000000100000002 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88809e2d3700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff88809e2d3780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff88809e2d3800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                         ^
 ffff88809e2d3880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88809e2d3900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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