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? 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
