From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> commit ef575281b21e9a34dfae544a187c6aac2ae424a9 upstream. syzbot is reporting hung task at p9_fd_close() [1], for p9_mux_poll_stop() from p9_conn_destroy() from p9_fd_close() is failing to interrupt already started kernel_read() from p9_fd_read() from p9_read_work() and/or kernel_write() from p9_fd_write() from p9_write_work() requests. Since p9_socket_open() sets O_NONBLOCK flag, p9_mux_poll_stop() does not need to interrupt kernel_read()/kernel_write(). However, since p9_fd_open() does not set O_NONBLOCK flag, but pipe blocks unless signal is pending, p9_mux_poll_stop() needs to interrupt kernel_read()/kernel_write() when the file descriptor refers to a pipe. In other words, pipe file descriptor needs to be handled as if socket file descriptor. We somehow need to interrupt kernel_read()/kernel_write() on pipes. A minimal change, which this patch is doing, is to set O_NONBLOCK flag from p9_fd_open(), for O_NONBLOCK flag does not affect reading/writing of regular files. But this approach changes O_NONBLOCK flag on userspace- supplied file descriptors (which might break userspace programs), and O_NONBLOCK flag could be changed by userspace. It would be possible to set O_NONBLOCK flag every time p9_fd_read()/p9_fd_write() is invoked, but still remains small race window for clearing O_NONBLOCK flag. If we don't want to manipulate O_NONBLOCK flag, we might be able to surround kernel_read()/kernel_write() with set_thread_flag(TIF_SIGPENDING) and recalc_sigpending(). Since p9_read_work()/p9_write_work() works are processed by kernel threads which process global system_wq workqueue, signals could not be delivered from remote threads when p9_mux_poll_stop() from p9_conn_destroy() from p9_fd_close() is called. Therefore, calling set_thread_flag(TIF_SIGPENDING)/recalc_sigpending() every time would be needed if we count on signals for making kernel_read()/kernel_write() non-blocking. Link: https://lkml.kernel.org/r/345de429-a88b-7097-d177-adecf9fed342@xxxxxxxxxxxxxxxxxxx Link: https://syzkaller.appspot.com/bug?extid=8b41a1365f1106fd0f33 [1] Reported-by: syzbot <syzbot+8b41a1365f1106fd0f33@xxxxxxxxxxxxxxxxxxxxxxxxx> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Tested-by: syzbot <syzbot+8b41a1365f1106fd0f33@xxxxxxxxxxxxxxxxxxxxxxxxx> Reviewed-by: Christian Schoenebeck <linux_oss@xxxxxxxxxxxxx> [Dominique: add comment at Christian's suggestion] Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- net/9p/trans_fd.c | 3 +++ 1 file changed, 3 insertions(+) --- a/net/9p/trans_fd.c +++ b/net/9p/trans_fd.c @@ -824,11 +824,14 @@ static int p9_fd_open(struct p9_client * goto out_free_ts; if (!(ts->rd->f_mode & FMODE_READ)) goto out_put_rd; + /* prevent workers from hanging on IO when fd is a pipe */ + ts->rd->f_flags |= O_NONBLOCK; ts->wr = fget(wfd); if (!ts->wr) goto out_put_rd; if (!(ts->wr->f_mode & FMODE_WRITE)) goto out_put_wr; + ts->wr->f_flags |= O_NONBLOCK; client->trans = ts; client->status = Connected;
