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SubjectRe: KASAN: use-after-free Write in __xfrm_policy_unlink
From
syzbot has found a reproducer for the following crash on:

HEAD commit: ce28bb445388 Merge git://git.kernel.org/pub/scm/linux/kern..
git tree: net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1673fb1b400000
kernel config: https://syzkaller.appspot.com/x/.config?x=67a2081147a23142
dashboard link: https://syzkaller.appspot.com/bug?extid=9d971dd21eb26567036b
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1134dcc7400000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=126986ed400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+9d971dd21eb26567036b@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in __hlist_del include/linux/list.h:704 [inline]
BUG: KASAN: use-after-free in hlist_del_rcu include/linux/rculist.h:455
[inline]
BUG: KASAN: use-after-free in __xfrm_policy_unlink+0xa09/0xa20
net/xfrm/xfrm_policy.c:2212
Write of size 8 at addr ffff8881ce732010 by task syz-executor247/6934

CPU: 0 PID: 6934 Comm: syz-executor247 Not tainted 4.20.0-rc7+ #358
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1d3/0x2c6 lib/dump_stack.c:113
print_address_description.cold.8+0x9/0x1ff mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report.cold.9+0x242/0x309 mm/kasan/report.c:412
__asan_report_store8_noabort+0x17/0x20 mm/kasan/report.c:438
__hlist_del include/linux/list.h:704 [inline]
hlist_del_rcu include/linux/rculist.h:455 [inline]
__xfrm_policy_unlink+0xa09/0xa20 net/xfrm/xfrm_policy.c:2212
xfrm_policy_insert+0x20a/0x850 net/xfrm/xfrm_policy.c:1587
xfrm_add_policy+0x2a0/0x6f0 net/xfrm/xfrm_user.c:1657
xfrm_user_rcv_msg+0x44c/0x8e0 net/xfrm/xfrm_user.c:2663
netlink_rcv_skb+0x16c/0x430 net/netlink/af_netlink.c:2477
xfrm_netlink_rcv+0x6f/0x90 net/xfrm/xfrm_user.c:2671
netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
netlink_unicast+0x59f/0x750 net/netlink/af_netlink.c:1336
netlink_sendmsg+0xa18/0xfc0 net/netlink/af_netlink.c:1917
sock_sendmsg_nosec net/socket.c:621 [inline]
sock_sendmsg+0xd5/0x120 net/socket.c:631
___sys_sendmsg+0x7fd/0x930 net/socket.c:2116
__sys_sendmsg+0x11d/0x280 net/socket.c:2154
__do_sys_sendmsg net/socket.c:2163 [inline]
__se_sys_sendmsg net/socket.c:2161 [inline]
__x64_sys_sendmsg+0x78/0xb0 net/socket.c:2161
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4448b9
Code: e8 dc b1 02 00 48 83 c4 18 c3 0f 1f 80 00 00 00 00 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 0f 83 db 01 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffd339e4118 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004448b9
RDX: 0000000000000000 RSI: 000000002014f000 RDI: 0000000000000004
RBP: 0000000000000000 R08: 00000000000002af R09: 00000000000002af
R10: 00000000000002af R11: 0000000000000246 R12: 00000000000002af
R13: 000000000000ae56 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 6932:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:553
kmem_cache_alloc_trace+0x152/0x750 mm/slab.c:3620
kmalloc include/linux/slab.h:546 [inline]
kzalloc include/linux/slab.h:741 [inline]
xfrm_policy_alloc+0xfa/0x4f0 net/xfrm/xfrm_policy.c:383
xfrm_policy_construct+0x2f/0x650 net/xfrm/xfrm_user.c:1599
xfrm_add_policy+0x203/0x6f0 net/xfrm/xfrm_user.c:1648
xfrm_user_rcv_msg+0x44c/0x8e0 net/xfrm/xfrm_user.c:2663
netlink_rcv_skb+0x16c/0x430 net/netlink/af_netlink.c:2477
xfrm_netlink_rcv+0x6f/0x90 net/xfrm/xfrm_user.c:2671
netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
netlink_unicast+0x59f/0x750 net/netlink/af_netlink.c:1336
netlink_sendmsg+0xa18/0xfc0 net/netlink/af_netlink.c:1917
sock_sendmsg_nosec net/socket.c:621 [inline]
sock_sendmsg+0xd5/0x120 net/socket.c:631
___sys_sendmsg+0x7fd/0x930 net/socket.c:2116
__sys_sendmsg+0x11d/0x280 net/socket.c:2154
__do_sys_sendmsg net/socket.c:2163 [inline]
__se_sys_sendmsg net/socket.c:2161 [inline]
__x64_sys_sendmsg+0x78/0xb0 net/socket.c:2161
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 16:
save_stack+0x43/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/kasan.c:521
kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
__cache_free mm/slab.c:3498 [inline]
kfree+0xcf/0x230 mm/slab.c:3817
xfrm_policy_destroy_rcu+0x4a/0x60 net/xfrm/xfrm_policy.c:407
__rcu_reclaim kernel/rcu/rcu.h:240 [inline]
rcu_do_batch kernel/rcu/tree.c:2437 [inline]
invoke_rcu_callbacks kernel/rcu/tree.c:2716 [inline]
rcu_process_callbacks+0x100a/0x1ac0 kernel/rcu/tree.c:2697
__do_softirq+0x30c/0xb2e kernel/softirq.c:292

The buggy address belongs to the object at ffff8881ce732000
which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 16 bytes inside of
1024-byte region [ffff8881ce732000, ffff8881ce732400)
The buggy address belongs to the page:
page:ffffea000739cc80 count:1 mapcount:0 mapping:ffff8881da800ac0 index:0x0
compound_mapcount: 0
flags: 0x2fffc0000010200(slab|head)
raw: 02fffc0000010200 ffffea000739d608 ffffea000739da88 ffff8881da800ac0
raw: 0000000000000000 ffff8881ce732000 0000000100000007 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8881ce731f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8881ce731f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8881ce732000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
^
ffff8881ce732080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8881ce732100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

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