CVE-2022-49767

In the Linux kernel, the following vulnerability has been resolved:

9p/trans_fd: always use O_NONBLOCK read/write

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 dont 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.

[Dominique: add comment at Christians suggestion]

Affected Software

References

• https://git.kernel.org/stable/c/0b5e6bd72b8171364616841603a70e4ba9837063
• https://git.kernel.org/stable/c/0e07032b4b4724b8ad1003698cb81083c1818999
• https://git.kernel.org/stable/c/5af16182c5639349415118e9e9aecd8355f7a08b
• https://git.kernel.org/stable/c/7abf40f06a76c0dff42eada10597917e9776fbd4
• https://git.kernel.org/stable/c/9f8554615df668e4bf83294633ee9d232b28ce45
• https://git.kernel.org/stable/c/a8e2fc8f7b41fa9d9ca5f624f4e4d34fce5b40a9
• https://git.kernel.org/stable/c/b1ad04da7fe4515e2ce2d5f2dcab3b5b6d45614b
• https://git.kernel.org/stable/c/ef575281b21e9a34dfae544a187c6aac2ae424a9