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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 ffff8801cde03868 by task syz-executor6/5496

CPU: 1 PID: 5496 Comm: syz-executor6 Not tainted 4.15.0-rc9+ #276
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_load8_noabort+0x14/0x20 mm/kasan/report.c:430
 shm_get_unmapped_area+0xfd/0x120 ipc/shm.c:478
 get_unmapped_area+0x18d/0x300 mm/mmap.c:2115
 do_mmap+0x2aa/0xe00 mm/mmap.c:1364
 do_mmap_pgoff include/linux/mm.h:2217 [inline]
 SYSC_remap_file_pages mm/mmap.c:2846 [inline]
 SyS_remap_file_pages+0x7ce/0xb30 mm/mmap.c:2762
 entry_SYSCALL_64_fastpath+0x29/0xa0
RIP: 0033:0x452f19
RSP: 002b:00007fca86e32c58 EFLAGS: 00000212 ORIG_RAX: 00000000000000d8
RAX: ffffffffffffffda RBX: 000000000071bea0 RCX: 0000000000452f19
RDX: 0000000000000000 RSI: 0000000000001000 RDI: 0000000020002000
RBP: 000000000000046e R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000212 R12: 00000000006f5af0
R13: 00000000ffffffff R14: 00007fca86e336d4 R15: 0000000000000000

Allocated by task 5496:
 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
 kmem_cache_zalloc include/linux/slab.h:678 [inline]
 get_empty_filp+0xfb/0x4f0 fs/file_table.c:123
 alloc_file+0x26/0x390 fs/file_table.c:164
 __shmem_file_setup+0x54f/0x6a0 mm/shmem.c:4223
 shmem_kernel_file_setup+0x32/0x40 mm/shmem.c:4249
 newseg+0x7bd/0xcb0 ipc/shm.c:589
 ipcget_new ipc/util.c:311 [inline]
 ipcget+0x8f0/0xfd0 ipc/util.c:649
 SYSC_shmget ipc/shm.c:675 [inline]
 SyS_shmget+0x158/0x230 ipc/shm.c:659
 entry_SYSCALL_64_fastpath+0x29/0xa0

Freed by task 5491:
 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
 file_free_rcu+0x5c/0x70 fs/file_table.c:50
 __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 ffff8801cde03840
 which belongs to the cache filp of size 456
The buggy address is located 40 bytes inside of
 456-byte region [ffff8801cde03840, ffff8801cde03a08)
The buggy address belongs to the page:
page:ffffea00073780c0 count:1 mapcount:0 mapping:ffff8801cde030c0 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801cde030c0 0000000000000000 0000000100000006
raw: ffffea0007360ee0 ffffea00073924e0 ffff8801dae2c180 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801cde03700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801cde03780: fb fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8801cde03800: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
                                                          ^
 ffff8801cde03880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff8801cde03900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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