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Date:   Sun, 23 Apr 2023 16:00:15 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        syzbot <syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com>,
        Jiri Slaby <jirislaby@kernel.org>,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4)
Message-ID: <2023042342-nebula-hydrogen-baf4@gregkh>
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On Sun, Apr 23, 2023 at 10:28:58PM +0900, Tetsuo Handa wrote:
> On 2023/04/21 17:21, Dmitry Vyukov wrote:
> > 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);
> 
> Hmm, it seems to me that we need multiple patches (which would become too big to
> backport) for fixing this bug.

Fix the bug properly first.  And then only start to worry about stable
kernels, don't let the existance of them affect your development efforts
at all please.

thanks,

greg k-h