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BUG: KASAN: use-after-free in erspan_build_header+0x3bf/0x3d0 net/ipv4/ip_gre.c:698
Read of size 2 at addr ffff8801c8e647cb by task syz-executor6/12472

CPU: 1 PID: 12472 Comm: syz-executor6 Not tainted 4.15.0-rc9+ #280
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report+0x25b/0x340 mm/kasan/report.c:409
 __asan_report_load_n_noabort+0xf/0x20 mm/kasan/report.c:440
 erspan_build_header+0x3bf/0x3d0 net/ipv4/ip_gre.c:698
 erspan_xmit+0x3b8/0x13b0 net/ipv4/ip_gre.c:740
 __netdev_start_xmit include/linux/netdevice.h:4042 [inline]
 netdev_start_xmit include/linux/netdevice.h:4051 [inline]
 packet_direct_xmit+0x315/0x6b0 net/packet/af_packet.c:266
 packet_snd net/packet/af_packet.c:2943 [inline]
 packet_sendmsg+0x3aed/0x60b0 net/packet/af_packet.c:2968
 sock_sendmsg_nosec net/socket.c:638 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:648
 SYSC_sendto+0x361/0x5c0 net/socket.c:1729
 SyS_sendto+0x40/0x50 net/socket.c:1697
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x452f19
RSP: 002b:00007fd911556c58 EFLAGS: 00000212 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 000000000071bf58 RCX: 0000000000452f19
RDX: 0000000000000000 RSI: 0000000020004000 RDI: 0000000000000014
RBP: 00000000000003d6 R08: 0000000020008000 R09: 000000000000001c
R10: 0000000000000000 R11: 0000000000000212 R12: 00000000006f4cb0
R13: 00000000ffffffff R14: 00007fd9115576d4 R15: 0000000000000002

Allocated by task 3707:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
 kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
 kmem_cache_alloc+0x12e/0x760 mm/slab.c:3544
 shmem_alloc_inode+0x1b/0x40 mm/shmem.c:3836
 alloc_inode+0x65/0x180 fs/inode.c:208
 new_inode_pseudo+0x69/0x190 fs/inode.c:890
 new_inode+0x1c/0x40 fs/inode.c:919
 shmem_get_inode+0xe1/0x920 mm/shmem.c:2157
 shmem_mknod+0x5a/0x1b0 mm/shmem.c:2990
 shmem_mkdir+0x29/0x50 mm/shmem.c:3041
 vfs_mkdir+0x390/0x600 fs/namei.c:3800
 SYSC_mkdirat fs/namei.c:3823 [inline]
 SyS_mkdirat fs/namei.c:3807 [inline]
 SYSC_mkdir fs/namei.c:3834 [inline]
 SyS_mkdir+0x220/0x2a0 fs/namei.c:3832
 entry_SYSCALL_64_fastpath+0x29/0xa0

Freed by task 8:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
 __cache_free mm/slab.c:3488 [inline]
 kmem_cache_free+0x83/0x2a0 mm/slab.c:3746
 shmem_destroy_callback+0x5a/0xa0 mm/shmem.c:3847
 __rcu_reclaim kernel/rcu/rcu.h:195 [inline]
 rcu_do_batch kernel/rcu/tree.c:2758 [inline]
 invoke_rcu_callbacks kernel/rcu/tree.c:3012 [inline]
 __rcu_process_callbacks kernel/rcu/tree.c:2979 [inline]
 rcu_process_callbacks+0xd6c/0x17f0 kernel/rcu/tree.c:2996
 __do_softirq+0x2d7/0xb85 kernel/softirq.c:285

The buggy address belongs to the object at ffff8801c8e64530
 which belongs to the cache shmem_inode_cache of size 1200
The buggy address is located 667 bytes inside of
 1200-byte region [ffff8801c8e64530, ffff8801c8e649e0)
The buggy address belongs to the page:
page:ffffea0007239900 count:1 mapcount:0 mapping:ffff8801c8e64000 index:0xffff8801c8e64ffd
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801c8e64000 ffff8801c8e64ffd 0000000100000003
raw: ffffea00071fd960 ffffea0007321320 ffff8801d9ea24c0 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801c8e64680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c8e64700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff8801c8e64780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                              ^
 ffff8801c8e64800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c8e64880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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