Re: [PATCH] tty: tty_io: fix race between tty_fops and hung_up_tty_fops

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Jul 22, 2024 at 06:10:41PM +0200, Jan Kara wrote:
> On Mon 22-07-24 16:41:22, Christian Brauner wrote:
> > On Fri, Jul 19, 2024 at 10:37:47PM GMT, Tetsuo Handa wrote:
> > > syzbot is reporting data race between __tty_hangup() and __fput(), and
> > > Dmitry Vyukov mentioned that this race has possibility of NULL pointer
> > > dereference, for tty_fops implements e.g. splice_read callback whereas
> > > hung_up_tty_fops does not.
> > > 
> > >   CPU0                                  CPU1
> > >   ----                                  ----
> > >   do_splice_read() {
> > >                                         __tty_hangup() {
> > >     // f_op->splice_read was copy_splice_read
> > >     if (unlikely(!in->f_op->splice_read))
> > >       return warn_unsupported(in, "read");
> > >                                           filp->f_op = &hung_up_tty_fops;
> > >     // f_op->splice_read is now NULL
> > >     return in->f_op->splice_read(in, ppos, pipe, len, flags);
> > >                                         }
> > >   }
> > > 
> > > Fix possibility of NULL pointer dereference by implementing missing
> > > callbacks, and suppress KCSAN messages by adding __data_racy qualifier
> > > to "struct file"->f_op .
> > 
> > This f_op replacing without synchronization seems really iffy imho.
> 
> Yeah, when I saw this I was also going "ouch". I was just waiting whether a
> tty maintainer will comment ;)

I really didn't want to :)

> Anyway this replacement of ops in file /
> inode has proven problematic in almost every single case where it was used
> leading to subtle issues.

Yeah, let's not do this.

Let me dig after -rc1 is out and see if there's a better way to handle
this contrived race condition...

thanks,

greg k-h