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BUG: KASAN: use-after-free in shm_get_unmapped_area+0xfd/0x120 ipc/shm.c:478
Read of size 8 at addr ffff8801c96c8a28 by task syz-executor2/4913

CPU: 1 PID: 4913 Comm: syz-executor2 Not tainted 4.15.0-rc6-next-20180105+ #89
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+0x137/0x198 lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report+0x23b/0x360 mm/kasan/report.c:412
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
 shm_get_unmapped_area+0xfd/0x120 ipc/shm.c:478
 get_unmapped_area+0x18d/0x300 mm/mmap.c:2126
 do_mmap+0x207/0xda0 mm/mmap.c:1368
 do_mmap_pgoff include/linux/mm.h:2224 [inline]
 SYSC_remap_file_pages mm/mmap.c:2857 [inline]
 SyS_remap_file_pages+0x74f/0x9f0 mm/mmap.c:2773
 entry_SYSCALL_64_fastpath+0x23/0x9a
RIP: 0033:0x452ac9
RSP: 002b:00007f5e18880c58 EFLAGS: 00000212 ORIG_RAX: 00000000000000d8
RAX: ffffffffffffffda RBX: 00007f5e18881700 RCX: 0000000000452ac9
RDX: 0000000000000000 RSI: 0000000000001000 RDI: 0000000020002000
RBP: 0000000000a2f870 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000212 R12: 0000000000000000
R13: 0000000000a2f7ef R14: 00007f5e188819c0 R15: 0000000000000004

Allocated by task 4895:
 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:552
 kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
 kmem_cache_alloc+0x12e/0x760 mm/slab.c:3541
 kmem_cache_zalloc include/linux/slab.h:694 [inline]
 get_empty_filp+0x8f/0x3c0 fs/file_table.c:122
 alloc_file+0x26/0x390 fs/file_table.c:163
 __shmem_file_setup.part.44+0x22f/0x420 mm/shmem.c:4230
 __shmem_file_setup mm/shmem.c:4200 [inline]
 shmem_kernel_file_setup+0x4f/0x70 mm/shmem.c:4256
 newseg+0x78d/0xc20 ipc/shm.c:589
 ipcget_new ipc/util.c:314 [inline]
 ipcget+0x6dd/0xb60 ipc/util.c:652
 SYSC_shmget ipc/shm.c:675 [inline]
 SyS_shmget+0xdf/0x140 ipc/shm.c:659
 entry_SYSCALL_64_fastpath+0x23/0x9a

Freed by task 3697:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:520
 kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:527
 __cache_free mm/slab.c:3485 [inline]
 kmem_cache_free+0x86/0x2b0 mm/slab.c:3743
 file_free_rcu+0x5c/0x70 fs/file_table.c:49
 __rcu_reclaim kernel/rcu/rcu.h:172 [inline]
 rcu_do_batch kernel/rcu/tree.c:2675 [inline]
 invoke_rcu_callbacks kernel/rcu/tree.c:2934 [inline]
 __rcu_process_callbacks kernel/rcu/tree.c:2901 [inline]
 rcu_process_callbacks+0x5cf/0x1200 kernel/rcu/tree.c:2918
 __do_softirq+0x23f/0x99f kernel/softirq.c:285

The buggy address belongs to the object at ffff8801c96c8a00
 which belongs to the cache filp of size 456
The buggy address is located 40 bytes inside of
 456-byte region [ffff8801c96c8a00, ffff8801c96c8bc8)
The buggy address belongs to the page:
page:ffffea000725b200 count:1 mapcount:0 mapping:ffff8801c96c8000 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801c96c8000 0000000000000000 0000000100000006
raw: ffffea0007253020 ffffea000725b320 ffff8801db230180 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801c96c8900: fb fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc
 ffff8801c96c8980: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8801c96c8a00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                  ^
 ffff8801c96c8a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801c96c8b00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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