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. 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. 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
