linux-kernel - Re: [PATCH net-next v7 1/4] scm: add SO_PASSPIDFD and SCM

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Date:   Fri, 1 Sep 2023 22:05:17 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Cc:     davem@...emloft.net, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        David Ahern <dsahern@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Christian Brauner <brauner@...nel.org>,
        Kuniyuki Iwashima <kuniyu@...zon.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Luca Boccassi <bluca@...ian.org>, linux-arch@...r.kernel.org,
        "Dmitry V. Levin" <ldv@...ace.io>
Subject: Re: [PATCH net-next v7 1/4] scm: add SO_PASSPIDFD and SCM_PIDFD

On Thu, Jun 08, 2023 at 10:26:25PM +0200, Alexander Mikhalitsyn wrote:
> Implement SCM_PIDFD, a new type of CMSG type analogical to SCM_CREDENTIALS,
> but it contains pidfd instead of plain pid, which allows programmers not
> to care about PID reuse problem.
> 
> We mask SO_PASSPIDFD feature if CONFIG_UNIX is not builtin because
> it depends on a pidfd_prepare() API which is not exported to the kernel
> modules.
> 
> Idea comes from UAPI kernel group:
> https://uapi-group.org/kernel-features/
> 
> Big thanks to Christian Brauner and Lennart Poettering for productive
> discussions about this.
> 
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: Leon Romanovsky <leon@...nel.org>
> Cc: David Ahern <dsahern@...nel.org>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Kuniyuki Iwashima <kuniyu@...zon.com>
> Cc: Lennart Poettering <mzxreary@...inter.de>
> Cc: Luca Boccassi <bluca@...ian.org>
> Cc: linux-kernel@...r.kernel.org
> Cc: netdev@...r.kernel.org
> Cc: linux-arch@...r.kernel.org
> Tested-by: Luca Boccassi <bluca@...ian.org>
> Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> Reviewed-by: Christian Brauner <brauner@...nel.org>
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
> ---
>  arch/alpha/include/uapi/asm/socket.h    |  2 ++
>  arch/mips/include/uapi/asm/socket.h     |  2 ++
>  arch/parisc/include/uapi/asm/socket.h   |  2 ++
>  arch/sparc/include/uapi/asm/socket.h    |  2 ++
>  include/linux/net.h                     |  1 +
>  include/linux/socket.h                  |  1 +
>  include/net/scm.h                       | 39 +++++++++++++++++++++++--
>  include/uapi/asm-generic/socket.h       |  2 ++
>  net/core/sock.c                         | 11 +++++++
>  net/mptcp/sockopt.c                     |  1 +
>  net/unix/af_unix.c                      | 18 ++++++++----
>  tools/include/uapi/asm-generic/socket.h |  2 ++
>  12 files changed, 76 insertions(+), 7 deletions(-)
...
> +static __inline__ void scm_pidfd_recv(struct msghdr *msg, struct scm_cookie *scm)
> +{
> +	struct file *pidfd_file = NULL;
> +	int pidfd;
> +
> +	/*
> +	 * put_cmsg() doesn't return an error if CMSG is truncated,
> +	 * that's why we need to opencode these checks here.
> +	 */
> +	if ((msg->msg_controllen <= sizeof(struct cmsghdr)) ||
> +	    (msg->msg_controllen - sizeof(struct cmsghdr)) < sizeof(int)) {
> +		msg->msg_flags |= MSG_CTRUNC;
> +		return;
> +	}

This does not work for compat tasks since the size of struct cmsghdr (aka
struct compat_cmsghdr) is differently. If the check from put_cmsg() is
open-coded here, then also a different check for compat tasks needs to be
added.

Discovered this because I was wondering why strace compat tests fail; it
seems because of this.

See https://github.com/strace/strace/blob/master/tests/scm_pidfd.c

For compat tasks recvmsg() returns with msg_flags=MSG_CTRUNC since the
above code expects a larger buffer than is necessary.