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

[Greg KH]

On Tue, May 02, 2017 at 11:52:40PM +0200, Vegard Nossum wrote:
> On 2 May 2017 at 18:35, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > On Fri, Apr 14, 2017 at 2:30 PM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >> On Fri, Apr 14, 2017 at 11:41:26AM +0200, Vegard Nossum wrote:
> >>> On 13 April 2017 at 20:34, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx> wrote:
> >>> 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?
> >
> > Vegard, do you know how to do this?
> > That was first thing that I tried, but I did not manage to make it
> > work. disc is tied to tty, so it's not that one can create a fully
> > initialized disc on the side and then simply swap pointers. Looking at
> > the code now, there is at least TTY_LDISC_OPEN bit in tty. But as far
> > as I remember there were more fundamental problems. Or maybe I just
> > did not try too hard.
> 
> I had a look at it but like you said, the tty/ldisc relationship is
> complicated :-/
> 
> Maybe we can split up ldisc initialisation into two methods so that
> the first one (e.g. ->alloc) does all the allocation and is allowed to
> fail and the second one (e.g. ->open) is not allowed to fail. Then you
> can allocate a new ldisc without freeing the old one and only swap
> them over if the allocation succeeded.
> 
> That would require fixing up ->open for all the ldisc drivers though,
> I'm not sure how easy/feasible it is.

We don't have that many ldisc drivers, so it shouldn't be that hard to
change to use this.  It makes a lot more sense to fix this the correct
way like this.

> I'll think about possible solutions, but I have no prior experience
> with the tty code. In the meantime syzkaller also hit a couple of
> other fun tty/pty bugs including a write/ioctl race that results in
> buffer overflow :-/

Ugh, let me know what they are and we'll work to fix them.

Thanks for all of the work you all are doing in finding these issues, I
might grumble about having to fix this and what a pain it is, but it's
just me being grumpy about the tty code, not your effort.  Your effort
is much appreciated.

thanks,

greg k-h