Re: [GIT PULL] TTY/Serial driver fixes for 4.11-rc4

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Apr 14, 2017 at 11:41:26AM +0200, Vegard Nossum wrote:
> On 13 April 2017 at 20:34, Greg KH <gregkh@linuxfoundation.org> wrote:
> > On Thu, Apr 13, 2017 at 09:07:40AM -0700, Linus Torvalds wrote:
> >> On Thu, Apr 13, 2017 at 3:50 AM, Vegard Nossum <vegard.nossum@gmail.com> wrote:
> >> >
> >> > I've bisected a syzkaller crash down to this commit
> >> > (5362544bebe85071188dd9e479b5a5040841c895). The crash is:
> >> >
> >> > [   25.137552] BUG: unable to handle kernel paging request at 0000000000002280
> >> > [   25.137579] IP: mutex_lock_interruptible+0xb/0x30
> >>
> >> It would seem to be the
> >>
> >>                 if (mutex_lock_interruptible(&ldata->atomic_read_lock))
> >>
> >> call in n_tty_read(), the offset is about right for a NULL 'ldata'
> >> pointer (it's a big structure, it has a couple of character buffers of
> >> size N_TTY_BUF_SIZE).
> >>
> >> I don't see the obvious fix, so I suspect at this point we should just
> >> revert, as that commit seems to introduce worse problems that it is
> >> supposed to fix. Greg?
> >
> > Unless Dmitry has a better idea, I will just revert it and send you the
> > pull request in a day or so.
> 
> I don't think we need to rush a revert, I'd hope there's a way to fix
> it properly.

For this late in the release cycle, for something as complex as tty
ldisc handling, for an issue that has been present for over a decade,
the safest thing right now is to go back to the old well-known code by
applying a revert :)

> So the original problem is that the vmalloc() in n_tty_open() can
> fail, and that will panic in tty_set_ldisc()/tty_ldisc_restore()
> because of its unwillingness to proceed if the tty doesn't have an
> ldisc.
> 
> Dmitry fixed this by allowing tty->ldisc == NULL in the case of memory
> allocation failure as we can see from the comment in tty_set_ldisc().
> 
> Unfortunately, it would appear that some other bits of code do not
> like tty->ldisc == NULL (other than the crash in this thread, I saw
> 2-3 similar crashes in other functions, e.g. poll()). I see two
> possibilities:
> 
> 1) make other code handle tty->ldisc == NULL.
> 
> 2) don't close/free the old ldisc until the new one has been
> successfully created/initialised/opened/attached to the tty, and
> return an error to userspace if changing it failed.
> 
> I'm leaning towards #2 as the more obviously correct fix, it makes
> tty_set_ldisc() transactional, the fix seems limited in scope to
> tty_set_ldisc() itself, and we don't need to make every other bit of
> code that uses tty->ldisc handle the NULL case.

That sounds reasonable to me, care to work on a patch for this?

thanks,

greg k-h