[net-next,v7,1/4] scm: add SO_PASSPIDFD and SCM_PIDFD

Commit Message

Alexander Mikhalitsyn June 8, 2023, 8:26 p.m. UTC

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" <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: Paolo Abeni <[e-mail]>
Cc: Leon Romanovsky <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Kuniyuki Iwashima <[e-mail]>
Cc: Lennart Poettering <[e-mail]>
Cc: Luca Boccassi <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Tested-by: Luca Boccassi <[e-mail]>
Reviewed-by: Kuniyuki Iwashima <[e-mail]>
Reviewed-by: Christian Brauner <[e-mail]>
Signed-off-by: Alexander Mikhalitsyn <[e-mail]>
---
v7:
	- removed CONFIG_UNIX checks, because we've converted CONFIG_UNIX to be boolean
v6:
	- disable feature when CONFIG_UNIX=n/m (pidfd_prepare API is not exported to modules)
v5:
	- no changes
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(-)

Comments

Eric Dumazet June 12, 2023, 9:19 a.m. UTC | #1

On Thu, Jun 8, 2023 at 10:26 PM Alexander Mikhalitsyn
<[e-mail]> 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" <[e-mail]>
> Cc: Eric Dumazet <[e-mail]>
> Cc: Jakub Kicinski <[e-mail]>
> Cc: Paolo Abeni <[e-mail]>
> Cc: Leon Romanovsky <[e-mail]>
> Cc: David Ahern <[e-mail]>
> Cc: Arnd Bergmann <[e-mail]>
> Cc: Kees Cook <[e-mail]>
> Cc: Christian Brauner <[e-mail]>
> Cc: Kuniyuki Iwashima <[e-mail]>
> Cc: Lennart Poettering <[e-mail]>
> Cc: Luca Boccassi <[e-mail]>
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> Tested-by: Luca Boccassi <[e-mail]>
> Reviewed-by: Kuniyuki Iwashima <[e-mail]>
> Reviewed-by: Christian Brauner <[e-mail]>
> Signed-off-by: Alexander Mikhalitsyn <[e-mail]>
> ---

Reviewed-by: Eric Dumazet <[e-mail]>

Alexander Mikhalitsyn June 12, 2023, 9:26 a.m. UTC | #2

On Mon, Jun 12, 2023 at 11:19 AM Eric Dumazet <[e-mail]> wrote:
>
> On Thu, Jun 8, 2023 at 10:26 PM Alexander Mikhalitsyn
> <[e-mail]> 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" <[e-mail]>
> > Cc: Eric Dumazet <[e-mail]>
> > Cc: Jakub Kicinski <[e-mail]>
> > Cc: Paolo Abeni <[e-mail]>
> > Cc: Leon Romanovsky <[e-mail]>
> > Cc: David Ahern <[e-mail]>
> > Cc: Arnd Bergmann <[e-mail]>
> > Cc: Kees Cook <[e-mail]>
> > Cc: Christian Brauner <[e-mail]>
> > Cc: Kuniyuki Iwashima <[e-mail]>
> > Cc: Lennart Poettering <[e-mail]>
> > Cc: Luca Boccassi <[e-mail]>
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Tested-by: Luca Boccassi <[e-mail]>
> > Reviewed-by: Kuniyuki Iwashima <[e-mail]>
> > Reviewed-by: Christian Brauner <[e-mail]>
> > Signed-off-by: Alexander Mikhalitsyn <[e-mail]>
> > ---
>
> Reviewed-by: Eric Dumazet <[e-mail]>

Thanks, Eric!

Kind regards,
Alex

Heiko Carstens Sept. 1, 2023, 8:05 p.m. UTC | #3

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" <[e-mail]>
> Cc: Eric Dumazet <[e-mail]>
> Cc: Jakub Kicinski <[e-mail]>
> Cc: Paolo Abeni <[e-mail]>
> Cc: Leon Romanovsky <[e-mail]>
> Cc: David Ahern <[e-mail]>
> Cc: Arnd Bergmann <[e-mail]>
> Cc: Kees Cook <[e-mail]>
> Cc: Christian Brauner <[e-mail]>
> Cc: Kuniyuki Iwashima <[e-mail]>
> Cc: Lennart Poettering <[e-mail]>
> Cc: Luca Boccassi <[e-mail]>
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
> Tested-by: Luca Boccassi <[e-mail]>
> Reviewed-by: Kuniyuki Iwashima <[e-mail]>
> Reviewed-by: Christian Brauner <[e-mail]>
> Signed-off-by: Alexander Mikhalitsyn <[e-mail]>
> ---
>  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.

Kuniyuki Iwashima Sept. 1, 2023, 8:33 p.m. UTC | #4

From: Heiko Carstens <[e-mail]>
Date: Fri, 1 Sep 2023 22:05:17 +0200
> 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" <[e-mail]>
> > Cc: Eric Dumazet <[e-mail]>
> > Cc: Jakub Kicinski <[e-mail]>
> > Cc: Paolo Abeni <[e-mail]>
> > Cc: Leon Romanovsky <[e-mail]>
> > Cc: David Ahern <[e-mail]>
> > Cc: Arnd Bergmann <[e-mail]>
> > Cc: Kees Cook <[e-mail]>
> > Cc: Christian Brauner <[e-mail]>
> > Cc: Kuniyuki Iwashima <[e-mail]>
> > Cc: Lennart Poettering <[e-mail]>
> > Cc: Luca Boccassi <[e-mail]>
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Tested-by: Luca Boccassi <[e-mail]>
> > Reviewed-by: Kuniyuki Iwashima <[e-mail]>
> > Reviewed-by: Christian Brauner <[e-mail]>
> > Signed-off-by: Alexander Mikhalitsyn <[e-mail]>
> > ---
> >  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.

Can you test this ?

---8<---
diff --git a/include/net/scm.h b/include/net/scm.h
index c5bcdf65f55c..099497ce4aee 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h
@@ -9,6 +9,7 @@
 #include <linux/pid.h>
 #include <linux/nsproxy.h>
 #include <linux/sched/signal.h>
+#include <net/compat.h>
 
 /* Well, we should have at least one descriptor open
  * to accept passed FDs 8)
@@ -125,14 +126,19 @@ 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,
+	/* 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;
+	if (msg->msg_flags & MSG_CMSG_COMPAT) {
+		if (msg->msg_controllen < sizeof(struct compat_cmsghdr) + sizeof(int)) {
+			msg->msg_flags |= MSG_CTRUNC;
+			return;
+		}
+	} else {
+		if (msg->msg_controllen < sizeof(struct cmsghdr) + sizeof(int)) {
+			msg->msg_flags |= MSG_CTRUNC;
+			return;
+		}
 	}
 
 	if (!scm->pid)
---8<---

Heiko Carstens Sept. 1, 2023, 8:51 p.m. UTC | #5

On Fri, Sep 01, 2023 at 01:33:22PM -0700, Kuniyuki Iwashima wrote:
> From: Heiko Carstens <[e-mail]>
> Date: Fri, 1 Sep 2023 22:05:17 +0200
> > On Thu, Jun 08, 2023 at 10:26:25PM +0200, Alexander Mikhalitsyn wrote:
> > > +	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.
> 
> Can you test this ?

Works for me.

Tested-by: Heiko Carstens <[e-mail]>

Kuniyuki Iwashima Sept. 1, 2023, 8:56 p.m. UTC | #6

From: Heiko Carstens <[e-mail]>
Date: Fri, 1 Sep 2023 22:51:45 +0200
> On Fri, Sep 01, 2023 at 01:33:22PM -0700, Kuniyuki Iwashima wrote:
> > From: Heiko Carstens <[e-mail]>
> > Date: Fri, 1 Sep 2023 22:05:17 +0200
> > > On Thu, Jun 08, 2023 at 10:26:25PM +0200, Alexander Mikhalitsyn wrote:
> > > > +	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.
> > 
> > Can you test this ?
> 
> Works for me.
> 
> Tested-by: Heiko Carstens <[e-mail]>

Thanks!
I'll post a formal patch.

Alexander Mikhalitsyn Sept. 1, 2023, 9:11 p.m. UTC | #7

On Fri, Sep 1, 2023 at 10:56 PM Kuniyuki Iwashima <[e-mail]> wrote:
>
> From: Heiko Carstens <[e-mail]>
> Date: Fri, 1 Sep 2023 22:51:45 +0200
> > On Fri, Sep 01, 2023 at 01:33:22PM -0700, Kuniyuki Iwashima wrote:
> > > From: Heiko Carstens <[e-mail]>
> > > Date: Fri, 1 Sep 2023 22:05:17 +0200
> > > > On Thu, Jun 08, 2023 at 10:26:25PM +0200, Alexander Mikhalitsyn wrote:
> > > > > +       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.
> > >
> > > Can you test this ?
> >
> > Works for me.
> >
> > Tested-by: Heiko Carstens <[e-mail]>

Reviewed-by: Alexander Mikhalitsyn <[e-mail]>

Thanks for reporting this, Heiko!
My bad.

Kuniyuki,
Thanks for the quick fix.

Kind regards,
Alex

>
> Thanks!
> I'll post a formal patch.

Patch

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 bd1cc3238851..3451a08f70d1 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;
+	}
+
+	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 (pidfd_file) {
+			put_unused_fd(pidfd);
+			fput(pidfd_file);
+		}
+
+		return;
+	}
+
+	if (pidfd_file)
+		fd_install(pidfd, pidfd_file);
+}
+
 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 +173,9 @@  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))
+		scm_pidfd_recv(msg, scm);
+
 	scm_destroy_cred(scm);
 
 	scm_passec(sock, msg, scm);
diff --git a/include/uapi/asm-generic/socket.h b/include/uapi/asm-generic/socket.h
index 638230899e98..b76169fdb80b 100644
--- a/include/uapi/asm-generic/socket.h
+++ b/include/uapi/asm-generic/socket.h
@@ -132,6 +132,8 @@ 
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))
diff --git a/net/core/sock.c b/net/core/sock.c
index 24f2761bdb1d..ed4eb4ba738b 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1246,6 +1246,13 @@  int sk_setsockopt(struct sock *sk, int level, int optname,
 			clear_bit(SOCK_PASSCRED, &sock->flags);
 		break;
 
+	case SO_PASSPIDFD:
+		if (valbool)
+			set_bit(SOCK_PASSPIDFD, &sock->flags);
+		else
+			clear_bit(SOCK_PASSPIDFD, &sock->flags);
+		break;
+
 	case SO_TIMESTAMP_OLD:
 	case SO_TIMESTAMP_NEW:
 	case SO_TIMESTAMPNS_OLD:
@@ -1732,6 +1739,10 @@  int sk_getsockopt(struct sock *sk, int level, int optname,
 		v.val = !!test_bit(SOCK_PASSCRED, &sock->flags);
 		break;
 
+	case SO_PASSPIDFD:
+		v.val = !!test_bit(SOCK_PASSPIDFD, &sock->flags);
+		break;
+
 	case SO_PEERCRED:
 	{
 		struct ucred peercred;
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index d4258869ac48..e172a5848b0d 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -355,6 +355,7 @@  static int mptcp_setsockopt_sol_socket(struct mptcp_sock *msk, int optname,
 	case SO_BROADCAST:
 	case SO_BSDCOMPAT:
 	case SO_PASSCRED:
+	case SO_PASSPIDFD:
 	case SO_PASSSEC:
 	case SO_RXQ_OVFL:
 	case SO_WIFI_STATUS:
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 653136d68b32..c46c2f5d860c 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1361,7 +1361,8 @@  static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
 		if (err)
 			goto out;
 
-		if (test_bit(SOCK_PASSCRED, &sock->flags) &&
+		if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
+		     test_bit(SOCK_PASSPIDFD, &sock->flags)) &&
 		    !unix_sk(sk)->addr) {
 			err = unix_autobind(sk);
 			if (err)
@@ -1469,7 +1470,8 @@  static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
 	if (err)
 		goto out;
 
-	if (test_bit(SOCK_PASSCRED, &sock->flags) && !u->addr) {
+	if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
+	     test_bit(SOCK_PASSPIDFD, &sock->flags)) && !u->addr) {
 		err = unix_autobind(sk);
 		if (err)
 			goto out;
@@ -1670,6 +1672,8 @@  static void unix_sock_inherit_flags(const struct socket *old,
 {
 	if (test_bit(SOCK_PASSCRED, &old->flags))
 		set_bit(SOCK_PASSCRED, &new->flags);
+	if (test_bit(SOCK_PASSPIDFD, &old->flags))
+		set_bit(SOCK_PASSPIDFD, &new->flags);
 	if (test_bit(SOCK_PASSSEC, &old->flags))
 		set_bit(SOCK_PASSSEC, &new->flags);
 }
@@ -1819,8 +1823,10 @@  static bool unix_passcred_enabled(const struct socket *sock,
 				  const struct sock *other)
 {
 	return test_bit(SOCK_PASSCRED, &sock->flags) ||
+	       test_bit(SOCK_PASSPIDFD, &sock->flags) ||
 	       !other->sk_socket ||
-	       test_bit(SOCK_PASSCRED, &other->sk_socket->flags);
+	       test_bit(SOCK_PASSCRED, &other->sk_socket->flags) ||
+	       test_bit(SOCK_PASSPIDFD, &other->sk_socket->flags);
 }
 
 /*
@@ -1904,7 +1910,8 @@  static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 			goto out;
 	}
 
-	if (test_bit(SOCK_PASSCRED, &sock->flags) && !u->addr) {
+	if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
+	     test_bit(SOCK_PASSPIDFD, &sock->flags)) && !u->addr) {
 		err = unix_autobind(sk);
 		if (err)
 			goto out;
@@ -2718,7 +2725,8 @@  static int unix_stream_read_generic(struct unix_stream_read_state *state,
 			/* Never glue messages from different writers */
 			if (!unix_skb_scm_eq(skb, &scm))
 				break;
-		} else if (test_bit(SOCK_PASSCRED, &sock->flags)) {
+		} else if (test_bit(SOCK_PASSCRED, &sock->flags) ||
+			   test_bit(SOCK_PASSPIDFD, &sock->flags)) {
 			/* Copy credentials */
 			scm_set_cred(&scm, UNIXCB(skb).pid, UNIXCB(skb).uid, UNIXCB(skb).gid);
 			unix_set_secdata(&scm, skb);
diff --git a/tools/include/uapi/asm-generic/socket.h b/tools/include/uapi/asm-generic/socket.h
index 8756df13be50..fbbc4bf53ee3 100644
--- a/tools/include/uapi/asm-generic/socket.h
+++ b/tools/include/uapi/asm-generic/socket.h
@@ -121,6 +121,8 @@ 
 
 #define SO_RCVMARK		75
 
+#define SO_PASSPIDFD		76
+
 #if !defined(__KERNEL__)
 
 #if __BITS_PER_LONG == 64 || (defined(__x86_64__) && defined(__ILP32__))