Re: [PATCH net-next v4 1/4] scm: add SO_PASSPIDFD and SCM_PIDFD

From: Christian Brauner
Date: Mon Apr 17 2023 - 11:19:17 EST

On Thu, Apr 13, 2023 at 03:33:52PM +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.
>
> 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: Luca Boccassi <bluca@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: linux-arch@xxxxxxxxxxxxxxx
> Tested-by: Luca Boccassi <bluca@xxxxxxxxxx>
> Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@xxxxxxxxxxxxx>
> ---
> v4:
> - fixed silent fd_install if writting of CMSG to the userspace fails (pointed by Christian)
> 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 | 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(-)
>
> 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..c67f765a165b 100644
> --- a/include/net/scm.h
> +++ b/include/net/scm.h
> @@ -120,12 +120,44 @@ static inline bool scm_has_secdata(struct socket *sock)
> }
> #endif /* CONFIG_SECURITY_NETWORK */
>
> +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;

Hm, curious about this: We mark the message as truncated for SCM_PIDFD
but if the same conditions were to apply for SCM_PASSCRED we don't mark
the message as truncated. Am I reading this correct? And is so, you
please briefly explain this difference?

> + }
> +
> + WARN_ON_ONCE(!scm->pid);
> + pidfd = pidfd_prepare(scm->pid, 0, &pidfd_file);
> +
> + if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {

If the put_cmsg() of the pidfd fails userspace needs to be able to
detect this. Otherwise they can't distinguish between the SCM_PIDFD
value being zero because the put_cmsg() failed or put_cmsg() succeeded
and the allocated fd nr was 0.

Looking at put_cmsg() it looks to me that userspace will receive a
SCM_PIDFD message only if the put_cmsg() is completely successful. IIUC,
then this change is fine.