On Tue, May 2, 2017 at 11:52 PM, 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. 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 -- 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
