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BUG: KASAN: use-after-free in memset include/linux/string.h:337 [inline]
BUG: KASAN: use-after-free in __ext4_expand_extra_isize+0x16c/0x240 fs/ext4/inode.c:5914
Write of size 512 at addr ffff888080461e80 by task syz-executor.1/7655

CPU: 1 PID: 7655 Comm: syz-executor.1 Not tainted 5.0.0-next-20190306 #4
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+0x172/0x1f0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
 kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x123/0x190 mm/kasan/generic.c:191
 memset+0x24/0x40 mm/kasan/common.c:115
 memset include/linux/string.h:337 [inline]
 __ext4_expand_extra_isize+0x16c/0x240 fs/ext4/inode.c:5914
 ext4_try_to_expand_extra_isize fs/ext4/inode.c:5966 [inline]
 ext4_mark_inode_dirty+0x6e9/0x940 fs/ext4/inode.c:6042
 ext4_dirty_inode+0x8f/0xc0 fs/ext4/inode.c:6076
 __mark_inode_dirty+0x919/0x1290 fs/fs-writeback.c:2155
 generic_update_time+0x21c/0x370 fs/inode.c:1651
 update_time fs/inode.c:1667 [inline]
 touch_atime+0x23e/0x2a0 fs/inode.c:1738
 get_link fs/namei.c:1055 [inline]
 trailing_symlink+0x2d4/0x970 fs/namei.c:2261
 path_mountpoint+0x868/0x1e60 fs/namei.c:2694
 filename_mountpoint+0x190/0x3c0 fs/namei.c:2715
 user_path_mountpoint_at+0x3a/0x50 fs/namei.c:2745
 ksys_umount+0x167/0xf00 fs/namespace.c:1687
 __do_sys_umount fs/namespace.c:1713 [inline]
 __se_sys_umount fs/namespace.c:1711 [inline]
 __x64_sys_umount+0x54/0x80 fs/namespace.c:1711
 do_syscall_64+0x103/0x610 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45a977
Code: 44 00 00 b8 08 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 4d 8e fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 b8 a6 00 00 00 0f 05 <48> 3d 01 f0 ff ff 0f 83 2d 8e fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffeeba4c488 EFLAGS: 00000206 ORIG_RAX: 00000000000000a6
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 000000000045a977
RDX: 0000000000402e80 RSI: 0000000000000002 RDI: 00007ffeeba4c530
RBP: 00000000000007e4 R08: 0000000000000000 R09: 000000000000000c
R10: 0000000000000006 R11: 0000000000000206 R12: 00007ffeeba4d5c0
R13: 0000000002377940 R14: 0000000000000000 R15: 00007ffeeba4d5c0

The buggy address belongs to the page:
page:ffffea0002011840 count:2 mapcount:0 mapping:ffff88809e064b18 index:0x436
def_blk_aops 
flags: 0x1fffc000000203a(referenced|dirty|lru|active|private)
raw: 01fffc000000203a ffffea000202b7c8 ffffea0001d34308 ffff88809e064b18
raw: 0000000000000436 ffff888095d3fe70 00000002ffffffff ffff888055ea8d00
page dumped because: kasan: bad access detected
page->mem_cgroup:ffff888055ea8d00

Memory state around the buggy address:
 ffff888080461f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff888080461f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff888080462000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff888080462080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff888080462100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
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