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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Fri, 21 Apr 2023 10:21:31 +0200
Message-ID: <CACT4Y+bj7jCOjGV5jCUPzN5zjgdHxRn9vkwQEBuPWVzMbMCnXw@mail.gmail.com>
Subject: Re: [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4)
To:     syzbot <syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com>
Cc:     gregkh@linuxfoundation.org, jirislaby@kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
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On Fri, 21 Apr 2023 at 10:18, syzbot
<syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    fcd476ea6a88 Merge tag 'urgent-rcu.2023.03.28a' of git://g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=146618b9c80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f7350c77b8056a38
> dashboard link: https://syzkaller.appspot.com/bug?extid=b7c3ba8cdc2f6cf83c21
> compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/f3d8ce4faab0/disk-fcd476ea.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/fc53d9dee279/vmlinux-fcd476ea.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/22ad755d39b2/bzImage-fcd476ea.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com

If I am reading this correctly, this race can lead to NULL derefs
among other things.
hung_up_tty_fops does not have splice_read, while other fops have.

So the following code in splice can execute NULL callback:

if (unlikely(!in->f_op->splice_read))
    return warn_unsupported(in, "read");
return in->f_op->splice_read(in, ppos, pipe, len, flags);


> ==================================================================
> BUG: KCSAN: data-race in __fput / __tty_hangup
>
> write to 0xffff88813b31e828 of 8 bytes by task 17702 on cpu 0:
>  __tty_hangup+0x1e2/0x510 drivers/tty/tty_io.c:622
>  tty_vhangup+0x17/0x20 drivers/tty/tty_io.c:701
>  pty_close+0x262/0x280 drivers/tty/pty.c:79
>  tty_release+0x204/0x930 drivers/tty/tty_io.c:1753
>  __fput+0x245/0x570 fs/file_table.c:321
>  ____fput+0x15/0x20 fs/file_table.c:349
>  task_work_run+0x123/0x160 kernel/task_work.c:179
>  get_signal+0xe5c/0xfe0 kernel/signal.c:2635
>  arch_do_signal_or_restart+0x89/0x2b0 arch/x86/kernel/signal.c:306
>  exit_to_user_mode_loop+0x6d/0xe0 kernel/entry/common.c:168
>  exit_to_user_mode_prepare+0x6a/0xa0 kernel/entry/common.c:204
>  __syscall_exit_to_user_mode_work kernel/entry/common.c:286 [inline]
>  syscall_exit_to_user_mode+0x26/0x140 kernel/entry/common.c:297
>  do_syscall_64+0x4d/0xc0 arch/x86/entry/common.c:86
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> read to 0xffff88813b31e828 of 8 bytes by task 17705 on cpu 1:
>  __fput+0x21c/0x570 fs/file_table.c:320
>  ____fput+0x15/0x20 fs/file_table.c:349
>  task_work_run+0x123/0x160 kernel/task_work.c:179
>  get_signal+0xe5c/0xfe0 kernel/signal.c:2635
>  arch_do_signal_or_restart+0x89/0x2b0 arch/x86/kernel/signal.c:306
>  exit_to_user_mode_loop+0x6d/0xe0 kernel/entry/common.c:168
>  exit_to_user_mode_prepare+0x6a/0xa0 kernel/entry/common.c:204
>  __syscall_exit_to_user_mode_work kernel/entry/common.c:286 [inline]
>  syscall_exit_to_user_mode+0x26/0x140 kernel/entry/common.c:297
>  do_syscall_64+0x4d/0xc0 arch/x86/entry/common.c:86
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> value changed: 0xffffffff84e91ed0 -> 0xffffffff84e91dc0
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 17705 Comm: syz-executor.0 Not tainted 6.3.0-rc4-syzkaller-00034-gfcd476ea6a88 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/17/2023
> ==================================================================
>
>
> ---
> This report 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 issue. See:
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