On Fri, Oct 25, 2019 at 12:04 AM Jens Axboe <axboe@xxxxxxxxx> wrote: > On 10/24/19 2:31 PM, Jann Horn wrote: > > On Thu, Oct 24, 2019 at 9:41 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >> On 10/18/19 12:50 PM, Jann Horn wrote: > >>> On Fri, Oct 18, 2019 at 8:16 PM Jens Axboe <axboe@xxxxxxxxx> wrote: > >>>> On 10/18/19 12:06 PM, Jann Horn wrote: > >>>>> But actually, by the way: Is this whole files_struct thing creating a > >>>>> reference loop? The files_struct has a reference to the uring file, > >>>>> and the uring file has ACCEPT work that has a reference to the > >>>>> files_struct. If the task gets killed and the accept work blocks, the > >>>>> entire files_struct will stay alive, right? > >>>> > >>>> Yes, for the lifetime of the request, it does create a loop. So if the > >>>> application goes away, I think you're right, the files_struct will stay. > >>>> And so will the io_uring, for that matter, as we depend on the closing > >>>> of the files to do the final reap. > >>>> > >>>> Hmm, not sure how best to handle that, to be honest. We need some way to > >>>> break the loop, if the request never finishes. > >>> > >>> A wacky and dubious approach would be to, instead of taking a > >>> reference to the files_struct, abuse f_op->flush() to synchronously > >>> flush out pending requests with references to the files_struct... But > >>> it's probably a bad idea, given that in f_op->flush(), you can't > >>> easily tell which files_struct the close is coming from. I suppose you > >>> could keep a list of (fdtable, fd) pairs through which ACCEPT requests > >>> have come in and then let f_op->flush() probe whether the file > >>> pointers are gone from them... > >> > >> Got back to this after finishing the io-wq stuff, which we need for the > >> cancel. > >> > >> Here's an updated patch: > >> > >> http://git.kernel.dk/cgit/linux-block/commit/?h=for-5.5/io_uring-test&id=1ea847edc58d6a54ca53001ad0c656da57257570 > >> > >> that seems to work for me (lightly tested), we correctly find and cancel > >> work that is holding on to the file table. > >> > >> The full series sits on top of my for-5.5/io_uring-wq branch, and can be > >> viewed here: > >> > >> http://git.kernel.dk/cgit/linux-block/log/?h=for-5.5/io_uring-test > >> > >> Let me know what you think! > > > > Ah, I didn't realize that the second argument to f_op->flush is a > > pointer to the files_struct. That's neat. > > > > > > Security: There is no guarantee that ->flush() will run after the last > > io_uring_enter() finishes. You can race like this, with threads A and > > B in one process and C in another one: > > > > A: sends uring fd to C via unix domain socket > > A: starts syscall io_uring_enter(fd, ...) > > A: calls fdget(fd), takes reference to file > > B: starts syscall close(fd) > > B: fd table entry is removed > > B: f_op->flush is invoked and finds no pending transactions > > B: syscall close() returns > > A: continues io_uring_enter(), grabbing current->files > > A: io_uring_enter() returns > > A and B: exit > > worker: use-after-free access to files_struct > > > > I think the solution to this would be (unless you're fine with adding > > some broad global read-write mutex) something like this in > > __io_queue_sqe(), where "fd" and "f" are the variables from > > io_uring_enter(), plumbed through the stack somehow: > > > > if (req->flags & REQ_F_NEED_FILES) { > > rcu_read_lock(); > > spin_lock_irq(&ctx->inflight_lock); > > if (fcheck(fd) == f) { > > list_add(&req->inflight_list, > > &ctx->inflight_list); > > req->work.files = current->files; > > ret = 0; > > } else { > > ret = -EBADF; > > } > > spin_unlock_irq(&ctx->inflight_lock); > > rcu_read_unlock(); > > if (ret) > > goto put_req; > > } > > First of all, thanks for the thorough look at this! We already have f > available here, it's req->file. And we just made a copy of the sqe, so > we have sqe->fd available as well. I fixed this up. sqe->fd is the file descriptor we're doing I/O on, not the file descriptor of the uring file, right? Same thing for req->file. This check only detects whether the fd we're doing I/O on was closed, which is irrelevant. > > Security + Correctness: If there is more than one io_wqe, it seems to > > me that io_uring_flush() calls io_wq_cancel_work(), which calls > > io_wqe_cancel_work(), which may return IO_WQ_CANCEL_OK if the first > > request it looks at is pending. In that case, io_wq_cancel_work() will > > immediately return, and io_uring_flush() will also immediately return. > > It looks like any other requests will continue running? > > Ah good point, I missed that. We need to keep looping until we get > NOTFOUND returned. Fixed as well. > > Also added cancellation if the task is going away. Here's the > incremental patch, I'll resend with the full version. [...] > +static int io_uring_flush(struct file *file, void *data) > +{ > + struct io_ring_ctx *ctx = file->private_data; > + > + if (fatal_signal_pending(current) || (current->flags & PF_EXITING)) > + io_wq_cancel_all(ctx->io_wq); Looking at io_wq_cancel_all(), this will just send a signal to the task without waiting for anything, right? Isn't that unsafe? > + else > + io_uring_cancel_files(ctx, data); > return 0; > }
