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    Subjectkernel BUG at fs/userfaultfd.c:LINE! (2)
    From
    Hello,

    syzbot found the following crash on:

    HEAD commit: 37b5dca2898d Merge tag 'rtc-4.18-2' of git://git.kernel.or..
    git tree: upstream
    console output: https://syzkaller.appspot.com/x/log.txt?x=12aaf3b2400000
    kernel config: https://syzkaller.appspot.com/x/.config?x=25856fac4e580aa7
    dashboard link: https://syzkaller.appspot.com/bug?extid=121be635a7a35ddb7dcb
    compiler: gcc (GCC) 8.0.1 20180413 (experimental)
    syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=11176d62400000
    C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1362b368400000

    IMPORTANT: if you fix the bug, please add the following tag to the commit:
    Reported-by: syzbot+121be635a7a35ddb7dcb@syzkaller.appspotmail.com

    random: sshd: uninitialized urandom read (32 bytes read)
    random: sshd: uninitialized urandom read (32 bytes read)
    random: sshd: uninitialized urandom read (32 bytes read)
    ------------[ cut here ]------------
    kernel BUG at fs/userfaultfd.c:883!
    invalid opcode: 0000 [#1] SMP KASAN
    CPU: 0 PID: 4708 Comm: syz-executor086 Not tainted 4.18.0-rc4+ #148
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
    Google 01/01/2011
    RIP: 0010:userfaultfd_release+0x5fb/0x770 fs/userfaultfd.c:882
    Code: 00 31 c0 48 8b 4d d0 65 48 33 0c 25 28 00 00 00 0f 85 a1 00 00 00 48
    8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d c3 e8 75 be a0 ff <0f> 0b 4c 89 f7
    e8 0b 9e de ff e9 11 fc ff ff 4c 89 e7 e8 5e 9d de
    RSP: 0018:ffff8801d8407a80 EFLAGS: 00010293
    RAX: ffff8801d8742080 RBX: dffffc0000000000 RCX: ffffffff81db4a59
    RDX: 0000000000000000 RSI: ffffffff81db4ccb RDI: 0000000000000000
    RBP: ffff8801d8407bd0 R08: ffff8801d8742080 R09: 0000000000000000
    R10: ffff8801d87428b8 R11: 16fb0df9f74494d6 R12: ffff8801d89a3b58
    R13: 0000000000000000 R14: ffff8801d89a3a50 R15: 0000000000000000
    FS: 0000000000ed6880(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 00007f45c6b81e78 CR3: 00000001acb61000 CR4: 00000000001406f0
    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
    Call Trace:
    __fput+0x355/0x8b0 fs/file_table.c:209
    ____fput+0x15/0x20 fs/file_table.c:243
    task_work_run+0x1ec/0x2a0 kernel/task_work.c:113
    tracehook_notify_resume include/linux/tracehook.h:192 [inline]
    exit_to_usermode_loop+0x313/0x370 arch/x86/entry/common.c:166
    prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
    syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
    do_syscall_64+0x6be/0x820 arch/x86/entry/common.c:293
    entry_SYSCALL_64_after_hwframe+0x49/0xbe
    RIP: 0033:0x405471
    Code: 75 14 b8 03 00 00 00 0f 05 48 3d 01 f0 ff ff 0f 83 54 17 00 00 c3 48
    83 ec 08 e8 6a fc ff ff 48 89 04 24 b8 03 00 00 00 0f 05 <48> 8b 3c 24 48
    89 c2 e8 b3 fc ff ff 48 89 d0 48 83 c4 08 48 3d 01
    RSP: 002b:00007ffcad3db600 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
    RAX: 0000000000000000 RBX: 0000000000000005 RCX: 0000000000405471
    RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000005
    RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
    R10: 00007ffcad3db610 R11: 0000000000000293 R12: 00000000006dbc3c
    R13: 00000000006dbda0 R14: 0000000000000008 R15: 0000000000000001
    Modules linked in:
    Dumping ftrace buffer:
    (ftrace buffer empty)
    ---[ end trace 638e68cedadb8eda ]---
    RIP: 0010:userfaultfd_release+0x5fb/0x770 fs/userfaultfd.c:882
    Code: 00 31 c0 48 8b 4d d0 65 48 33 0c 25 28 00 00 00 0f 85 a1 00 00 00 48
    8d 65 d8 5b 41 5c 41 5d 41 5e 41 5f 5d c3 e8 75 be a0 ff <0f> 0b 4c 89 f7
    e8 0b 9e de ff e9 11 fc ff ff 4c 89 e7 e8 5e 9d de
    RSP: 0018:ffff8801d8407a80 EFLAGS: 00010293
    RAX: ffff8801d8742080 RBX: dffffc0000000000 RCX: ffffffff81db4a59
    RDX: 0000000000000000 RSI: ffffffff81db4ccb RDI: 0000000000000000
    RBP: ffff8801d8407bd0 R08: ffff8801d8742080 R09: 0000000000000000
    R10: ffff8801d87428b8 R11: 16fb0df9f74494d6 R12: ffff8801d89a3b58
    R13: 0000000000000000 R14: ffff8801d89a3a50 R15: 0000000000000000
    FS: 0000000000ed6880(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 00007f45c6b81e78 CR3: 00000001acb61000 CR4: 00000000001406f0
    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


    ---
    This bug is generated by a bot. It may contain errors.
    See https://goo.gl/tpsmEJ for more information about syzbot.
    syzbot engineers can be reached at syzkaller@googlegroups.com.

    syzbot will keep track of this bug report. See:
    https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with
    syzbot.
    syzbot can test patches for this bug, for details see:
    https://goo.gl/tpsmEJ#testing-patches

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