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: >> >> > >> >> > 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? 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. -- 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
