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Subject: Re: possible deadlock in tty_unthrottle
To:     Hillf Danton <hdanton@sina.com>,
        syzbot <syzbot+26183d9746e62da329b8@syzkaller.appspotmail.com>
Cc:     gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
References: <20200227113622.4716-1-hdanton@sina.com>
From:   Jiri Slaby <jslaby@suse.com>
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Message-ID: <2dd19dd1-abd7-9a6d-356b-4806841c2671@suse.com>
Date:   Thu, 27 Feb 2020 12:40:10 +0100
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On 27. 02. 20, 12:36, Hillf Danton wrote:
> 
> On Thu, 27 Feb 2020 00:39:12 -0800
>> syzbot found the following crash on:
>>
>> HEAD commit:    f8788d86 Linux 5.6-rc3
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=1102d22de00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5d2e033af114153f
>> dashboard link: https://syzkaller.appspot.com/bug?extid=26183d9746e62da329b8
>> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
>>
>> Unfortunately, I don't have any reproducer for this crash yet.
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+26183d9746e62da329b8@syzkaller.appspotmail.com
>>
>> ======================================================
>> WARNING: possible circular locking dependency detected
>> 5.6.0-rc3-syzkaller #0 Not tainted
>> ------------------------------------------------------
>> syz-executor.4/20336 is trying to acquire lock:
>> ffff8880a2e952a0 (&tty->termios_rwsem){++++}, at: tty_unthrottle+0x22/0x100 drivers/tty/tty_ioctl.c:136
>>
>> but task is already holding lock:
>> ffffffff89462e70 (sel_lock){+.+.}, at: paste_selection+0x118/0x470 drivers/tty/vt/selection.c:374
>>
>> which lock already depends on the new lock.
>>
>>
>> the existing dependency chain (in reverse order) is:
>>
>> -> #2 (sel_lock){+.+.}:
>>        lock_acquire+0x154/0x250 kernel/locking/lockdep.c:4484
>>        __mutex_lock_common+0x16e/0x2f30 kernel/locking/mutex.c:956
>>        __mutex_lock kernel/locking/mutex.c:1103 [inline]
>>        mutex_lock_nested+0x1b/0x30 kernel/locking/mutex.c:1118
>>        set_selection_kernel+0x3b8/0x18a0 drivers/tty/vt/selection.c:217
>>        set_selection_user+0x63/0x80 drivers/tty/vt/selection.c:181
>>        tioclinux+0x103/0x530 drivers/tty/vt/vt.c:3050
>>        vt_ioctl+0x3f1/0x3a30 drivers/tty/vt/vt_ioctl.c:364
>>        tty_ioctl+0xee6/0x15c0 drivers/tty/tty_io.c:2660
>>        vfs_ioctl fs/ioctl.c:47 [inline]
>>        ksys_ioctl fs/ioctl.c:763 [inline]
>>        __do_sys_ioctl fs/ioctl.c:772 [inline]
>>        __se_sys_ioctl+0x113/0x190 fs/ioctl.c:770
>>        __x64_sys_ioctl+0x7b/0x90 fs/ioctl.c:770
>>        do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
>>        entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> -> #1 (console_lock){+.+.}:
>>        lock_acquire+0x154/0x250 kernel/locking/lockdep.c:4484
>>        console_lock+0x46/0x70 kernel/printk/printk.c:2289
>>        con_flush_chars+0x50/0x650 drivers/tty/vt/vt.c:3223
>>        n_tty_write+0xeae/0x1200 drivers/tty/n_tty.c:2350
>>        do_tty_write drivers/tty/tty_io.c:962 [inline]
>>        tty_write+0x5a1/0x950 drivers/tty/tty_io.c:1046
>>        __vfs_write+0xb8/0x740 fs/read_write.c:494
>>        vfs_write+0x270/0x580 fs/read_write.c:558
>>        ksys_write+0x117/0x220 fs/read_write.c:611
>>        __do_sys_write fs/read_write.c:623 [inline]
>>        __se_sys_write fs/read_write.c:620 [inline]
>>        __x64_sys_write+0x7b/0x90 fs/read_write.c:620
>>        do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
>>        entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> -> #0 (&tty->termios_rwsem){++++}:
>>        check_prev_add kernel/locking/lockdep.c:2475 [inline]
>>        check_prevs_add kernel/locking/lockdep.c:2580 [inline]
>>        validate_chain+0x1507/0x7be0 kernel/locking/lockdep.c:2970
>>        __lock_acquire+0xc5a/0x1bc0 kernel/locking/lockdep.c:3954
>>        lock_acquire+0x154/0x250 kernel/locking/lockdep.c:4484
>>        down_write+0x57/0x140 kernel/locking/rwsem.c:1534
>>        tty_unthrottle+0x22/0x100 drivers/tty/tty_ioctl.c:136
>>        mkiss_receive_buf+0x12aa/0x1340 drivers/net/hamradio/mkiss.c:902
>>        tty_ldisc_receive_buf+0x12f/0x170 drivers/tty/tty_buffer.c:465
>>        paste_selection+0x346/0x470 drivers/tty/vt/selection.c:389
>>        tioclinux+0x121/0x530 drivers/tty/vt/vt.c:3055
>>        vt_ioctl+0x3f1/0x3a30 drivers/tty/vt/vt_ioctl.c:364
>>        tty_ioctl+0xee6/0x15c0 drivers/tty/tty_io.c:2660
>>        vfs_ioctl fs/ioctl.c:47 [inline]
>>        ksys_ioctl fs/ioctl.c:763 [inline]
>>        __do_sys_ioctl fs/ioctl.c:772 [inline]
>>        __se_sys_ioctl+0x113/0x190 fs/ioctl.c:770
>>        __x64_sys_ioctl+0x7b/0x90 fs/ioctl.c:770
>>        do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
>>        entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>
>> other info that might help us debug this:
>>
>> Chain exists of:
>>   &tty->termios_rwsem --> console_lock --> sel_lock
>>
>>  Possible unsafe locking scenario:
>>
>>        CPU0                    CPU1
>>        ----                    ----
>>   lock(sel_lock);
>>                                lock(console_lock);
>>                                lock(sel_lock);
>>   lock(&tty->termios_rwsem);
>>
>>  *** DEADLOCK ***
>>
>> 3 locks held by syz-executor.4/20336:
>>  #0: ffff8880a2e95090 (&tty->ldisc_sem){++++}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:267
>>  #1: ffff888097ac10a8 (&buf->lock){+.+.}, at: tty_buffer_lock_exclusive+0x33/0x40 drivers/tty/tty_buffer.c:61
>>  #2: ffffffff89462e70 (sel_lock){+.+.}, at: paste_selection+0x118/0x470 drivers/tty/vt/selection.c:374
>>
> The sel_lock was introduced in
> 	07e6124a1a46 ("vt: selection, close sel_buffer race")
> 
> and with it in position can we cut one lock for setting selection?

Ouch -- I will look into it. (But not until tomorrow.)

Thanks.

> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -3015,8 +3015,6 @@ static struct console vt_console_driver
>   *
>   * There are some functions which can sleep for arbitrary periods
>   * (paste_selection) but we don't need the lock there anyway.
> - *
> - * set_selection_user has locking, and definitely needs it
>   */
>  
>  int tioclinux(struct tty_struct *tty, unsigned long arg)
> @@ -3035,10 +3033,8 @@ int tioclinux(struct tty_struct *tty, un
>  	switch (type)
>  	{
>  		case TIOCL_SETSEL:
> -			console_lock();
>  			ret = set_selection_user((struct tiocl_selection
>  						 __user *)(p+1), tty);
> -			console_unlock();
>  			break;
>  		case TIOCL_PASTESEL:
>  			ret = paste_selection(tty);
> 


-- 
js
suse labs