Re: [PATCH v4 6/6] net: tap: Use qemu_close_range() to close fds

[Jason Wang]

On Wed, Jun 28, 2023 at 11:29 PM Bin Meng <bmeng@tinylab.org> wrote:
>
> From: Zhangjin Wu <falcon@tinylab.org>
>
> Current codes using a brute-force traversal of all file descriptors
> do not scale on a system where the maximum number of file descriptors
> is set to a very large value (e.g.: in a Docker container of Manjaro
> distribution it is set to 1073741816). QEMU just looks frozen during
> start-up.
>
> The close-on-exec flag (O_CLOEXEC) was introduced since Linux kernel
> 2.6.23, FreeBSD 8.3, OpenBSD 5.0, Solaris 11. While it's true QEMU
> doesn't need to manually close the fds for child process as the proper
> O_CLOEXEC flag should have been set properly on files with its own
> codes, QEMU uses a huge number of 3rd party libraries and we don't
> trust them to reliably be using O_CLOEXEC on everything they open.
>
> Modern Linux and BSDs have the close_range() call we can use to do the
> job, and on Linux we have one more way to walk through /proc/self/fd
> to complete the task efficiently, which is what qemu_close_range() does.
>
> Reported-by: Zhangjin Wu <falcon@tinylab.org>
> Co-developed-by: Bin Meng <bmeng@tinylab.org>
> Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
> Signed-off-by: Bin Meng <bmeng@tinylab.org>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

Patch looks good but I'm not sure using helper scripts is good for the
production environment since it increases attack surfaces. Passing TAP
fd should be a better way.

Thanks

>
> ---
>
> Changes in v4:
> - put fd on its own line
>
> Changes in v2:
> - Change to use qemu_close_range() to close fds for child process efficiently
> - v1 link: 
> https://lore.kernel.org/qemu-devel/20230406112041.798585-1-bmeng@tinylab.org/
>
>  net/tap.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/net/tap.c b/net/tap.c
> index 1bf085d422..9f080215f0 100644
> --- a/net/tap.c
> +++ b/net/tap.c
> @@ -446,13 +446,13 @@ static void launch_script(const char *setup_script, 
> const char *ifname,
>          return;
>      }
>      if (pid == 0) {
> -        int open_max = sysconf(_SC_OPEN_MAX), i;
> +        unsigned int last_fd = sysconf(_SC_OPEN_MAX) - 1;
> +
> +        /* skip stdin, stdout and stderr */
> +        qemu_close_range(3, fd - 1);
> +        /* skip the currently used fd */
> +        qemu_close_range(fd + 1, last_fd);
>
> -        for (i = 3; i < open_max; i++) {
> -            if (i != fd) {
> -                close(i);
> -            }
> -        }
>          parg = args;
>          *parg++ = (char *)setup_script;
>          *parg++ = (char *)ifname;
> @@ -536,16 +536,16 @@ static int net_bridge_run_helper(const char *helper, 
> const char *bridge,
>          return -1;
>      }
>      if (pid == 0) {
> -        int open_max = sysconf(_SC_OPEN_MAX), i;
> +        unsigned int last_fd = sysconf(_SC_OPEN_MAX) - 1;
> +        unsigned int fd = sv[1];
>          char *fd_buf = NULL;
>          char *br_buf = NULL;
>          char *helper_cmd = NULL;
>
> -        for (i = 3; i < open_max; i++) {
> -            if (i != sv[1]) {
> -                close(i);
> -            }
> -        }
> +        /* skip stdin, stdout and stderr */
> +        qemu_close_range(3, fd - 1);
> +        /* skip the currently used fd */
> +        qemu_close_range(fd + 1, last_fd);
>
>          fd_buf = g_strdup_printf("%s%d", "--fd=", sv[1]);
>
> --
> 2.34.1
>