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 -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
