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

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Message-ID: <CACT4Y+ax=nq-4UJjjH5iFLXsygCYhdbup5G7Y1xrsTN4iQG3mQ@mail.gmail.com>
Date:   Wed, 3 May 2017 13:25:01 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Vegard Nossum <vegard.nossum@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-serial <linux-serial@...r.kernel.org>
Subject: Re: [GIT PULL] TTY/Serial driver fixes for 4.11-rc4

On Tue, May 2, 2017 at 11:52 PM, Vegard Nossum <vegard.nossum@...il.com> 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.


What do you think about making all tty code deal with NULL disc?
It seems that most of code is already prepared for 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 :-/
>
>
> Vegard