CVE-2022-49767 Information

May 02, 2025 cve

Description

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 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.

[Dominique: add comment at Christian’s suggestion]

Reference

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