Re: [PATCH net-next v2 1/3] scm: add SO_PASSPIDFD and SCM_PIDFD

[Christian Brauner]

On Tue, Mar 21, 2023 at 07:33:40PM +0100, 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.
> 
> 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@xxxxxxxxxxxxx>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> Cc: Leon Romanovsky <leon@xxxxxxxxxx>
> Cc: David Ahern <dsahern@xxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Christian Brauner <brauner@xxxxxxxxxx>
> Cc: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> Cc: Lennart Poettering <mzxreary@xxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: linux-arch@xxxxxxxxxxxxxxx
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
> ---
> v2:
> 	According to review comments from Kuniyuki Iwashima and Christian Brauner:
> 	- use pidfd_create(..) retval as a result
> 	- whitespace change
> ---
>  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                       | 14 ++++++++++++--
>  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, 51 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/alpha/include/uapi/asm/socket.h b/arch/alpha/include/uapi/asm/socket.h
> index 739891b94136..ff310613ae64 100644
> --- a/arch/alpha/include/uapi/asm/socket.h
> +++ b/arch/alpha/include/uapi/asm/socket.h
> @@ -137,6 +137,8 @@
>  
>  #define SO_RCVMARK		75
>  
> +#define SO_PASSPIDFD		76
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/mips/include/uapi/asm/socket.h b/arch/mips/include/uapi/asm/socket.h
> index 18f3d95ecfec..762dcb80e4ec 100644
> --- a/arch/mips/include/uapi/asm/socket.h
> +++ b/arch/mips/include/uapi/asm/socket.h
> @@ -148,6 +148,8 @@
>  
>  #define SO_RCVMARK		75
>  
> +#define SO_PASSPIDFD		76
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/parisc/include/uapi/asm/socket.h b/arch/parisc/include/uapi/asm/socket.h
> index f486d3dfb6bb..df16a3e16d64 100644
> --- a/arch/parisc/include/uapi/asm/socket.h
> +++ b/arch/parisc/include/uapi/asm/socket.h
> @@ -129,6 +129,8 @@
>  
>  #define SO_RCVMARK		0x4049
>  
> +#define SO_PASSPIDFD		0x404A
> +
>  #if !defined(__KERNEL__)
>  
>  #if __BITS_PER_LONG == 64
> diff --git a/arch/sparc/include/uapi/asm/socket.h b/arch/sparc/include/uapi/asm/socket.h
> index 2fda57a3ea86..6e2847804fea 100644
> --- a/arch/sparc/include/uapi/asm/socket.h
> +++ b/arch/sparc/include/uapi/asm/socket.h
> @@ -130,6 +130,8 @@
>  
>  #define SO_RCVMARK               0x0054
>  
> +#define SO_PASSPIDFD             0x0055
> +
>  #if !defined(__KERNEL__)
>  
>  
> diff --git a/include/linux/net.h b/include/linux/net.h
> index b73ad8e3c212..c234dfbe7a30 100644
> --- a/include/linux/net.h
> +++ b/include/linux/net.h
> @@ -43,6 +43,7 @@ struct net;
>  #define SOCK_PASSSEC		4
>  #define SOCK_SUPPORT_ZC		5
>  #define SOCK_CUSTOM_SOCKOPT	6
> +#define SOCK_PASSPIDFD		7
>  
>  #ifndef ARCH_HAS_SOCKET_TYPES
>  /**
> diff --git a/include/linux/socket.h b/include/linux/socket.h
> index 13c3a237b9c9..6bf90f251910 100644
> --- a/include/linux/socket.h
> +++ b/include/linux/socket.h
> @@ -177,6 +177,7 @@ static inline size_t msg_data_left(struct msghdr *msg)
>  #define	SCM_RIGHTS	0x01		/* rw: access rights (array of int) */
>  #define SCM_CREDENTIALS 0x02		/* rw: struct ucred		*/
>  #define SCM_SECURITY	0x03		/* rw: security label		*/
> +#define SCM_PIDFD	0x04		/* ro: pidfd (int)		*/
>  
>  struct ucred {
>  	__u32	pid;
> diff --git a/include/net/scm.h b/include/net/scm.h
> index 585adc1346bd..0c717ae9c8db 100644
> --- a/include/net/scm.h
> +++ b/include/net/scm.h
> @@ -124,8 +124,9 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
>  				struct scm_cookie *scm, int flags)
>  {
>  	if (!msg->msg_control) {
> -		if (test_bit(SOCK_PASSCRED, &sock->flags) || scm->fp ||
> -		    scm_has_secdata(sock))
> +		if (test_bit(SOCK_PASSCRED, &sock->flags) ||
> +		    test_bit(SOCK_PASSPIDFD, &sock->flags) ||
> +		    scm->fp || scm_has_secdata(sock))
>  			msg->msg_flags |= MSG_CTRUNC;
>  		scm_destroy(scm);
>  		return;
> @@ -141,6 +142,15 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
>  		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(ucreds), &ucreds);
>  	}
>  
> +	if (test_bit(SOCK_PASSPIDFD, &sock->flags)) {
> +		int pidfd;
> +
> +		WARN_ON_ONCE(!scm->pid);
> +		pidfd = pidfd_create(scm->pid, 0);
> +
> +		put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd);

So here we need to also make sure that we can't end up in a situation
where the receiver gets an error message and discards the message but
we've snuck an fd into their fdtable. So callers of scm_recv() should be
in a path where the message can't fail anymore and we're about to return
to userspace.