linux-kernel - Re: WARNING: lock held when returning to user space in fuse_lock

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Message-ID: <CAJfpeguHRhT4xwDNu-xEfA+YOrB7Wo7eM8sU19RGLaBpEN0g_A@mail.gmail.com>
Date:   Tue, 17 Jul 2018 13:14:28 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     syzbot <syzbot+3f7b29af1baa9d0a55be@...kaller.appspotmail.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: WARNING: lock held when returning to user space in fuse_lock_inode

On Thu, Jul 12, 2018 at 5:49 PM, syzbot
<syzbot+3f7b29af1baa9d0a55be@...kaller.appspotmail.com> wrote:
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    c25c74b7476e Merge tag 'trace-v4.18-rc3-2' of git://git.ke..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=177bcec2400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=25856fac4e580aa7
> dashboard link: https://syzkaller.appspot.com/bug?extid=3f7b29af1baa9d0a55be
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=13aa7678400000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17492678400000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+3f7b29af1baa9d0a55be@...kaller.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)
>
> ================================================
> WARNING: lock held when returning to user space!
> 4.18.0-rc4+ #143 Not tainted
> ------------------------------------------------
> syz-executor012/4539 is leaving the kernel with locks still held!
> 1 lock held by syz-executor012/4539:
>  #0: (____ptrval____) (&fi->mutex){+.+.}, at: fuse_lock_inode+0xaf/0xe0
> fs/fuse/inode.c:363

False positive.

fi->mutex is definitely not held by the acquiring task when returning
to userspace.  Maybe syzkaller is confused by the fact that there are
several interdependent tasks involved with fuse:  the one calling into
fuse by doing something (looking up ./file0/file0) and the one that
reads the fuse device (returning with the LOOKUP request for "file0").
The second one will return with that lock held, but it's not the one
that acquired it, so there's no bug at all here.

Thanks,
Miklos

>
>
> ---
> 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@...glegroups.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