syzbot


KASAN: use-after-free Read in hci_cmd_timeout

Status: upstream: reported C repro on 2019/05/07 09:10
Reported-by: syzbot+19a9f729f05272857487@syzkaller.appspotmail.com
First crash: 1316d, last: 6d17h

Cause bisection: introduced by (bisect log) :
commit ff92b9dd9268507e23fc10cc4341626cef50367c
Author: Suganath Prabu <suganath-prabu.subramani@broadcom.com>
Date: Thu Oct 25 14:03:40 2018 +0000

  scsi: mpt3sas: Update MPI headers to support Aero controllers

Crash: KASAN: use-after-free Read in hci_cmd_timeout (log)
Repro: C syz .config

Fix bisection: failed (bisect log)
similar bugs (2):
Kernel Title Repro Cause bisect Fix bisect Count Last Reported Patched Status
linux-4.19 KASAN: use-after-free Read in hci_cmd_timeout syz error 16 26d 1193d 0/1 upstream: reported syz repro on 2019/09/01 02:37
linux-4.14 KASAN: use-after-free Read in hci_cmd_timeout C inconclusive 11 323d 1187d 0/1 upstream: reported C repro on 2019/09/06 20:31
Patch testing requests:
Created Duration User Patch Repo Result
2021/08/14 15:41 18m phind.uet@gmail.com linux-next OK
2020/10/27 16:43 18m anmol.karan123@gmail.com upstream OK

Sample crash report:
Bluetooth: hci6: command tx timeout
==================================================================
BUG: KASAN: use-after-free in hci_cmd_timeout+0x203/0x210 net/bluetooth/hci_core.c:2654
Read of size 2 at addr ffff88806cf57c08 by task kworker/1:3/3749

CPU: 1 PID: 3749 Comm: kworker/1:3 Not tainted 5.16.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events hci_cmd_timeout
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247
 __kasan_report mm/kasan/report.c:433 [inline]
 kasan_report.cold+0x83/0xdf mm/kasan/report.c:450
 hci_cmd_timeout+0x203/0x210 net/bluetooth/hci_core.c:2654
 process_one_work+0x9b2/0x1660 kernel/workqueue.c:2298
 worker_thread+0x65d/0x1130 kernel/workqueue.c:2445
 kthread+0x405/0x4f0 kernel/kthread.c:327
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
 </TASK>

Allocated by task 3720:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 kasan_set_track mm/kasan/common.c:46 [inline]
 set_alloc_info mm/kasan/common.c:434 [inline]
 ____kasan_kmalloc mm/kasan/common.c:513 [inline]
 ____kasan_kmalloc mm/kasan/common.c:472 [inline]
 __kasan_kmalloc+0xa9/0xd0 mm/kasan/common.c:522
 kmalloc include/linux/slab.h:590 [inline]
 kzalloc include/linux/slab.h:724 [inline]
 kernfs_fop_open+0x2b9/0xd30 fs/kernfs/file.c:628
 do_dentry_open+0x4b9/0x1240 fs/open.c:822
 do_open fs/namei.c:3426 [inline]
 path_openat+0x1cad/0x2750 fs/namei.c:3559
 do_filp_open+0x1aa/0x400 fs/namei.c:3586
 do_sys_openat2+0x16d/0x4d0 fs/open.c:1212
 do_sys_open fs/open.c:1228 [inline]
 __do_sys_openat fs/open.c:1244 [inline]
 __se_sys_openat fs/open.c:1239 [inline]
 __x64_sys_openat+0x13f/0x1f0 fs/open.c:1239
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Freed by task 3703:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 kasan_set_track+0x21/0x30 mm/kasan/common.c:46
 kasan_set_free_info+0x20/0x30 mm/kasan/generic.c:370
 ____kasan_slab_free mm/kasan/common.c:366 [inline]
 ____kasan_slab_free mm/kasan/common.c:328 [inline]
 __kasan_slab_free+0xff/0x130 mm/kasan/common.c:374
 kasan_slab_free include/linux/kasan.h:235 [inline]
 slab_free_hook mm/slub.c:1723 [inline]
 slab_free_freelist_hook+0x8b/0x1c0 mm/slub.c:1749
 slab_free mm/slub.c:3513 [inline]
 kfree+0xf6/0x560 mm/slub.c:4561
 skb_free_head net/core/skbuff.c:655 [inline]
 skb_release_data+0x65a/0x790 net/core/skbuff.c:677
 skb_release_all net/core/skbuff.c:742 [inline]
 __kfree_skb net/core/skbuff.c:756 [inline]
 kfree_skb net/core/skbuff.c:774 [inline]
 kfree_skb+0x133/0x3f0 net/core/skbuff.c:768
 hci_dev_do_open+0xa50/0x1870 net/bluetooth/hci_core.c:1513
 hci_power_on+0x133/0x650 net/bluetooth/hci_core.c:2143
 process_one_work+0x9b2/0x1660 kernel/workqueue.c:2298
 worker_thread+0x65d/0x1130 kernel/workqueue.c:2445
 kthread+0x405/0x4f0 kernel/kthread.c:327
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

The buggy address belongs to the object at ffff88806cf57c00
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 8 bytes inside of
 512-byte region [ffff88806cf57c00, ffff88806cf57e00)
The buggy address belongs to the page:
page:ffffea0001b3d500 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x6cf54
head:ffffea0001b3d500 order:2 compound_mapcount:0 compound_pincount:0
flags: 0xfff00000010200(slab|head|node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000010200 ffffea0001e89200 dead000000000002 ffff888010c41c80
raw: 0000000000000000 0000000000100010 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 2, migratetype Unmovable, gfp_mask 0xd28c0(GFP_NOWAIT|__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 3697, ts 1610686922392, free_ts 12316497295
 prep_new_page mm/page_alloc.c:2418 [inline]
 get_page_from_freelist+0xa72/0x2f50 mm/page_alloc.c:4149
 __alloc_pages+0x1b2/0x500 mm/page_alloc.c:5369
 alloc_pages+0x1a7/0x300 mm/mempolicy.c:2190
 alloc_slab_page mm/slub.c:1793 [inline]
 allocate_slab mm/slub.c:1930 [inline]
 new_slab+0x32d/0x4a0 mm/slub.c:1993
 ___slab_alloc+0x918/0xfe0 mm/slub.c:3022
 __slab_alloc.constprop.0+0x4d/0xa0 mm/slub.c:3109
 slab_alloc_node mm/slub.c:3200 [inline]
 __kmalloc_node_track_caller+0x2cb/0x360 mm/slub.c:4956
 kmalloc_reserve net/core/skbuff.c:354 [inline]
 pskb_expand_head+0x15e/0x1060 net/core/skbuff.c:1701
 netlink_trim+0x1ea/0x240 net/netlink/af_netlink.c:1301
 netlink_broadcast+0x5b/0xd50 net/netlink/af_netlink.c:1497
 nlmsg_multicast include/net/netlink.h:1033 [inline]
 nlmsg_notify+0x8f/0x280 net/netlink/af_netlink.c:2539
 rtmsg_fib+0x2e3/0x450 net/ipv4/fib_semantics.c:530
 fib_table_insert+0x714/0x1af0 net/ipv4/fib_trie.c:1389
 fib_magic+0x455/0x540 net/ipv4/fib_frontend.c:1087
 fib_add_ifaddr+0x476/0x500 net/ipv4/fib_frontend.c:1129
 fib_netdev_event+0x46f/0x680 net/ipv4/fib_frontend.c:1466
page last free stack trace:
 reset_page_owner include/linux/page_owner.h:24 [inline]
 free_pages_prepare mm/page_alloc.c:1338 [inline]
 free_pcp_prepare+0x374/0x870 mm/page_alloc.c:1389
 free_unref_page_prepare mm/page_alloc.c:3309 [inline]
 free_unref_page+0x19/0x690 mm/page_alloc.c:3388
 free_contig_range+0xa8/0xf0 mm/page_alloc.c:9271
 destroy_args+0xa8/0x646 mm/debug_vm_pgtable.c:1016
 debug_vm_pgtable+0x2961/0x29f3 mm/debug_vm_pgtable.c:1330
 do_one_initcall+0x103/0x650 init/main.c:1297
 do_initcall_level init/main.c:1370 [inline]
 do_initcalls init/main.c:1386 [inline]
 do_basic_setup init/main.c:1405 [inline]
 kernel_init_freeable+0x6b1/0x73a init/main.c:1610
 kernel_init+0x1a/0x1d0 init/main.c:1499
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

Memory state around the buggy address:
 ffff88806cf57b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff88806cf57b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff88806cf57c00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                      ^
 ffff88806cf57c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88806cf57d00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Crashes (369):
Manager Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Title
ci-upstream-kasan-gce-root 2021/12/30 00:56 upstream e7c124bd0463 6cc879d4 .config log report syz C KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-386 2021/07/11 15:14 upstream 3dbdb38e2869 8f5a7b8c .config log report syz C KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/09/25 19:46 linux-next aaa11ce2ffc8 0042f2b4 .config log report syz C KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/08/25 22:16 linux-next b5d939c95186 9b5bf4cd .config log report syz C KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/08/24 21:59 linux-next 68a00424bf69 514514f6 .config log report syz C KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/05/15 03:41 linux-next 1e1b28b936ae 744a39e2 .config log report syz C KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2019/07/03 13:11 upstream eca94432934f 55565fa0 .config log report syz C
ci-upstream-kasan-gce-smack-root 2022/10/25 17:56 upstream a970174d7a10 45645420 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/10/03 08:21 upstream a962b54e162c feb56351 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/07/31 16:56 upstream 6a010258447d fef302b1 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/04/11 06:25 upstream 1862a69c9174 e22c3da3 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/03/24 06:20 upstream 1bc191051dca 5ff41e94 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/08/23 18:43 linux-next 05477f3653b8 cea8b0f7 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/03/12 22:23 linux-next 91265a6da44d 9e8eaa75 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/03/03 07:05 linux-next e6ada6df471f 45a13a73 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2021/12/13 07:21 linux-next ea922272cbe5 49ca1f59 .config log report syz KASAN: use-after-free Read in hci_cmd_timeout
ci-qemu-upstream 2020/06/16 19:24 upstream 435faf5c218a 4ea9d964 .config log report syz
ci-upstream-kasan-gce-selinux-root 2022/11/30 13:29 upstream 01f856ae6d0c 4c2a66e8 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/11/30 01:06 upstream 01f856ae6d0c 05dc7993 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/11/28 03:34 upstream bf82d38c91f8 74a66371 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/11/24 19:00 upstream c3eb11fbb826 62e26685 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/11/21 14:26 upstream 77c51ba552a1 5bb70014 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/11/21 10:50 upstream 77c51ba552a1 5bb70014 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/11/21 00:52 upstream 77c51ba552a1 5bb70014 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/11/19 11:26 upstream fe24a97cf254 5bb70014 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/11/17 05:33 upstream 59d0d52c30d4 3a127a31 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-qemu-upstream 2022/11/07 09:17 upstream f0c4d9fc9cc9 6d752409 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/11/05 13:57 upstream 10d916c86eca 6d752409 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/11/01 04:09 upstream b229b6ca5abb a1d8560a .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/10/30 19:59 upstream b229b6ca5abb 2a71366b .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/10/30 00:48 upstream b229b6ca5abb 2a71366b .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/10/27 08:21 upstream 98555239e4c3 86777b7f .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/10/27 00:07 upstream 98555239e4c3 86777b7f .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/10/22 23:18 upstream 4da34b7d175d c0b80a55 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/10/22 18:00 upstream 4da34b7d175d c0b80a55 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/10/22 15:56 upstream 4da34b7d175d c0b80a55 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/10/22 04:14 upstream e35184f32151 4bfd3c27 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/10/21 13:28 upstream e35184f32151 4bfd3c27 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/10/18 11:00 upstream 55be6084c8e0 754863b4 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/10/16 18:15 upstream 55be6084c8e0 67cb024c .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/10/16 01:49 upstream 55be6084c8e0 67cb024c .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/10/15 16:12 upstream 55be6084c8e0 67cb024c .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/10/14 08:41 upstream 55be6084c8e0 4954e4b2 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/10/13 19:52 upstream 55be6084c8e0 adf90437 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/10/13 09:24 upstream 55be6084c8e0 3f6b40a1 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/10/13 04:20 upstream 55be6084c8e0 3f6b40a1 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-qemu-upstream 2022/10/06 13:22 upstream 833477fce7a1 131b38ac .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/10/02 02:45 upstream b357fd1c2afc feb56351 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-qemu-upstream 2022/09/30 18:19 upstream 5a77386984b5 5e8ac358 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/09/28 16:42 upstream 49c13ed0316d e2556bc3 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/09/27 13:51 upstream a1375562c0a8 87840e00 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-smack-root 2022/09/24 16:33 upstream a63f2e7cb110 0042f2b4 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/09/23 11:35 upstream bf682942cd26 0042f2b4 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/09/20 18:06 upstream 521a547ced64 7c41a9ba .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/09/19 07:55 upstream 521a547ced64 dd9a85ff .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/09/18 03:23 upstream a335366bad13 dd9a85ff .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-root 2022/09/14 08:09 upstream d1221cea11fc b884348d .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/09/08 06:17 upstream 0066f1b0e275 435aeef7 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-selinux-root 2022/09/01 18:59 upstream 2880e1a175b9 86c46e46 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce 2021/10/24 08:09 upstream 9c0c4d24ac00 282f03fb .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-qemu-upstream-386 2022/10/28 21:20 upstream 200204f56f3b bc17b3a4 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-qemu-upstream-386 2022/10/17 09:32 upstream 9abf2313adc1 67cb024c .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-qemu-upstream-386 2022/09/09 15:15 upstream 9b4509495418 90058bdc .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce-386 2021/10/11 11:57 upstream 64570fbc14f8 838e7e2c .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/10/15 07:20 linux-next aaa11ce2ffc8 67cb024c .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/10/11 15:49 linux-next aaa11ce2ffc8 1353c374 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/10/10 02:52 linux-next aaa11ce2ffc8 aea5da89 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/10/07 07:49 linux-next aaa11ce2ffc8 8a212197 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/09/28 23:00 linux-next aaa11ce2ffc8 e2556bc3 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/09/27 06:16 linux-next aaa11ce2ffc8 10323ddf .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/09/18 17:22 linux-next e47eb90a0a9a dd9a85ff .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/09/16 02:44 linux-next e47eb90a0a9a dd9a85ff .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/09/07 03:58 linux-next e47eb90a0a9a 5fc30c37 .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-linux-next-kasan-gce-root 2022/08/30 00:31 linux-next b27a3ca08ba7 5b44472d .config log report info KASAN: use-after-free Read in hci_cmd_timeout
ci-upstream-kasan-gce 2021/01/14 09:52 upstream 65f0d2414b70 269d24e8 .config log report info
ci-upstream-kasan-gce 2019/04/30 13:29 upstream 83a50840e72a 20f16bef .config log report
ci-upstream-linux-next-kasan-gce-root 2022/01/13 00:15 linux-next 32ce2abb03cf 44d1319a .config log report info KFENCE: use-after-free in hci_cmd_timeout
* Struck through repros no longer work on HEAD.