Re: [syzbot] memory leak in ext4_multi_mount_protect

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit:    4fa56ad0 Merge tag 'for-linus' of git://git.kernel.org/pub..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12390a96d00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b8dbd3c72fdc7777
dashboard link: https://syzkaller.appspot.com/bug?extid=d9e482e303930fa4f6ff
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=109aaa7ed00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16e77d16d00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d9e482e303930fa4f6ff@xxxxxxxxxxxxxxxxxxxxxxxxx

executing program
BUG: memory leak
unreferenced object 0xffff888111edd780 (size 32):
  comm "syz-executor633", pid 8448, jiffies 4294951405 (age 17.620s)
  hex dump (first 32 bytes):
    10 64 d1 0f 81 88 ff ff 00 10 7e 12 81 88 ff ff  .d........~.....
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff81744606>] kmalloc include/linux/slab.h:554 [inline]
    [<ffffffff81744606>] ext4_multi_mount_protect+0x4a6/0x5d0 fs/ext4/mmp.c:367
    [<ffffffff81783614>] ext4_fill_super+0x56a4/0x5b20 fs/ext4/super.c:4769
    [<ffffffff8154ebf3>] mount_bdev+0x223/0x260 fs/super.c:1367
    [<ffffffff815aee7b>] legacy_get_tree+0x2b/0x90 fs/fs_context.c:592
    [<ffffffff8154c7e8>] vfs_get_tree+0x28/0x100 fs/super.c:1497
    [<ffffffff8158c8ee>] do_new_mount fs/namespace.c:2903 [inline]
    [<ffffffff8158c8ee>] path_mount+0xc3e/0x1120 fs/namespace.c:3233
    [<ffffffff8158d51e>] do_mount fs/namespace.c:3246 [inline]
    [<ffffffff8158d51e>] __do_sys_mount fs/namespace.c:3454 [inline]
    [<ffffffff8158d51e>] __se_sys_mount fs/namespace.c:3431 [inline]
    [<ffffffff8158d51e>] __x64_sys_mount+0x18e/0x1d0 fs/namespace.c:3431
    [<ffffffff842ded2d>] do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
    [<ffffffff84400068>] entry_SYSCALL_64_after_hwframe+0x44/0xae