CVE-2022-49767 in Linux
Summary
by MITRE • 05/01/2025
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]
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Analysis
by VulDB Data Team • 11/07/2025
The vulnerability CVE-2022-49767 affects the Linux kernel's 9p file system implementation, specifically within the fd transport layer. This issue manifests as a hung task condition that occurs during the cleanup process of file descriptor connections. The problem stems from improper handling of file descriptor blocking behavior when dealing with pipe-based connections versus socket-based connections. The kernel's 9p implementation maintains separate code paths for different transport mechanisms, but fails to consistently apply the same non-blocking I/O semantics across all file descriptor types.
The technical root cause lies in the inconsistent application of the O_NONBLOCK flag during file descriptor initialization. When socket-based connections are established through p9_socket_open(), the O_NONBLOCK flag is properly set, ensuring that read and write operations do not block indefinitely. However, the p9_fd_open() function does not apply this same flag to pipe-based file descriptors, creating a scenario where kernel_read() and kernel_write() operations can block indefinitely when processing pipe connections. This blocking behavior prevents proper cleanup operations from completing, resulting in hung tasks that can degrade system performance and potentially lead to system instability.
The operational impact of this vulnerability extends beyond simple performance degradation to potential system resource exhaustion and denial of service conditions. When the p9_mux_poll_stop() function attempts to interrupt ongoing kernel_read() and kernel_write() operations during connection teardown, it fails to properly handle pipe-based file descriptors that remain in blocking mode. This creates a deadlock scenario where cleanup operations cannot proceed, leading to resource leaks and potential system crashes. The vulnerability affects systems utilizing 9p file sharing protocols, particularly those implementing pipe-based transport mechanisms for communication between kernel and userspace components.
This vulnerability aligns with CWE-661, which addresses insufficient control flow management, and maps to ATT&CK technique T1499.004 for endpoint resource exhaustion. The issue represents a classic race condition and improper resource management problem where kernel threads fail to properly handle asynchronous I/O operations across different file descriptor types. The fix implemented in the patch involves consistently applying the O_NONBLOCK flag to all file descriptors opened through p9_fd_open(), ensuring uniform behavior across socket and pipe transport mechanisms. However, this approach introduces potential compatibility concerns with userspace applications that may depend on specific blocking behavior for their operations, creating a trade-off between system stability and application compatibility.
The mitigation strategy requires careful consideration of the O_NONBLOCK flag's impact on existing userspace programs that may rely on traditional blocking I/O semantics. Alternative approaches such as signal-based interruption mechanisms using set_thread_flag(TIF_SIGPENDING) and recalc_sigpending() could provide a more targeted solution but introduce additional complexity in signal delivery timing and thread management. The kernel developers must balance immediate system stability improvements with maintaining backward compatibility for legitimate userspace applications that depend on predictable I/O behavior. This vulnerability highlights the importance of consistent I/O handling across different transport mechanisms in kernel subsystems and demonstrates how seemingly minor inconsistencies in flag application can lead to significant operational impacts.