LinuxLists.cc - [PATCH net-next v4 1/4] scm: add SO_PASSPIDFD and SCM

2023-04-13 13:38:52

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

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" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Leon Romanovsky <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Kuniyuki Iwashima <[email protected]>
Cc: Lennart Poettering <[email protected]>
Cc: Luca Boccassi <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Tested-by: Luca Boccassi <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Signed-off-by: Alexander Mikhalitsyn <[email protected]>
---
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;
+ }
+
+ 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 c25888795390..3f974246ba3e 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:
@@ -1737,6 +1744,10 @@ 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 b655cebda0f3..67be0558862f 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 fb31e8a4409e..6d5dff4dfe83 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);
}

/*
@@ -1922,7 +1928,8 @@ 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;
@@ -2824,7 +2831,8 @@ 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__))
--
2.34.1

2023-04-17 15:21:14

by Christian Brauner

[permalink] [raw]

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

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

2023-04-17 16:02:55

by Aleksandr Mikhalitsyn

[permalink] [raw]

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

On Mon, Apr 17, 2023 at 5:18 PM Christian Brauner <[email protected]> wrote:
>
> 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" <[email protected]>
> > Cc: Eric Dumazet <[email protected]>
> > Cc: Jakub Kicinski <[email protected]>
> > Cc: Paolo Abeni <[email protected]>
> > Cc: Leon Romanovsky <[email protected]>
> > Cc: David Ahern <[email protected]>
> > Cc: Arnd Bergmann <[email protected]>
> > Cc: Kees Cook <[email protected]>
> > Cc: Christian Brauner <[email protected]>
> > Cc: Kuniyuki Iwashima <[email protected]>
> > Cc: Lennart Poettering <[email protected]>
> > Cc: Luca Boccassi <[email protected]>
> > Cc: [email protected]
> > Cc: [email protected]
> > Cc: [email protected]
> > Tested-by: Luca Boccassi <[email protected]>
> > Reviewed-by: Kuniyuki Iwashima <[email protected]>
> > Signed-off-by: Alexander Mikhalitsyn <[email protected]>
> > ---
> > 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?

Hi, Christian!

For SCM_CREDENTIALS we mark it too. Inside the put_cmsg function:
https://github.com/torvalds/linux/blob/6a8f57ae2eb07ab39a6f0ccad60c760743051026/net/core/scm.c#L225

The reason why I'm open-coding these checks is that I want to know
that the message
doesn't fit into the userspace buffer before doing pidfd_prepare and
other stuff and because
put_cmsg is not returning an error when message doesn't fit in the
userspace buffer and
we won't be able to properly do pidfd cleanup (put struct pid and fd index).

>
> > + }
> > +
> > + 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.

If pidfd_prepare fails then userspace will receive SCM_PIDFD message
with negative pidfd value.

>
> 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.

Kind regards,
Alex

2023-04-17 17:17:20

by Christian Brauner

[permalink] [raw]

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

On Mon, Apr 17, 2023 at 06:01:16PM +0200, Aleksandr Mikhalitsyn wrote:
> On Mon, Apr 17, 2023 at 5:18 PM Christian Brauner <[email protected]> wrote:
> >
> > 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" <[email protected]>
> > > Cc: Eric Dumazet <[email protected]>
> > > Cc: Jakub Kicinski <[email protected]>
> > > Cc: Paolo Abeni <[email protected]>
> > > Cc: Leon Romanovsky <[email protected]>
> > > Cc: David Ahern <[email protected]>
> > > Cc: Arnd Bergmann <[email protected]>
> > > Cc: Kees Cook <[email protected]>
> > > Cc: Christian Brauner <[email protected]>
> > > Cc: Kuniyuki Iwashima <[email protected]>
> > > Cc: Lennart Poettering <[email protected]>
> > > Cc: Luca Boccassi <[email protected]>
> > > Cc: [email protected]
> > > Cc: [email protected]
> > > Cc: [email protected]
> > > Tested-by: Luca Boccassi <[email protected]>
> > > Reviewed-by: Kuniyuki Iwashima <[email protected]>
> > > Signed-off-by: Alexander Mikhalitsyn <[email protected]>
> > > ---
> > > 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?
>
> Hi, Christian!
>
> For SCM_CREDENTIALS we mark it too. Inside the put_cmsg function:
> https://github.com/torvalds/linux/blob/6a8f57ae2eb07ab39a6f0ccad60c760743051026/net/core/scm.c#L225
>
> The reason why I'm open-coding these checks is that I want to know
> that the message
> doesn't fit into the userspace buffer before doing pidfd_prepare and
> other stuff and because
> put_cmsg is not returning an error when message doesn't fit in the
> userspace buffer and
> we won't be able to properly do pidfd cleanup (put struct pid and fd index).
>
> >
> > > + }
> > > +
> > > + 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.
>
> If pidfd_prepare fails then userspace will receive SCM_PIDFD message
> with negative pidfd value.

So we discussed this a bit offline and I think there's still an issue.
If put_cmsg() fails

if (msg->msg_control_is_user) {
struct cmsghdr __user *cm = msg->msg_control_user;

check_object_size(data, cmlen - sizeof(*cm), true);

if (!user_write_access_begin(cm, cmlen))
goto efault;

// This succeeds so cm->cmsg_len == sizeof(int)
unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);

// This succeeds so cm->cmsg_level == SOL_SOCKET
unsafe_put_user(level, &cm->cmsg_level, efault_end);

// This succeeds so cm->cmsg_type == SCM_PIDFD
unsafe_put_user(type, &cm->cmsg_type, efault_end);

// This fails and leaves all bits set to 0
unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
cmlen - sizeof(*cm), efault_end);
user_write_access_end();

so now we hit

if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {
if (pidfd_file) {
put_unused_fd(pidfd);
fput(pidfd_file);
}

return;
}

and return early. Afaict, userspace would now receive:

if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
cmsg->cmsg_level == SOL_SOCKET &&
cmsg->cmsg_type == SCM_PIDFD) {
memcpy(&pidfd, CMSG_DATA(cmsg), sizeof(int));

// pidfd is now 0 which is a valid fd number
// it'll likely refer to /dev/stdin or whatever and so
// will fail or, worst case, 0 refers to another pidfd :)
pidfd_send_signal(pidfd, SIGKILL);

so we need to address this. So one way I think that would solve this is:

diff --git a/net/core/scm.c b/net/core/scm.c
index 3cd7dd377e53..d1f4cd135c5a 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -236,9 +236,9 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)

unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);
unsafe_put_user(level, &cm->cmsg_level, efault_end);
- unsafe_put_user(type, &cm->cmsg_type, efault_end);
unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
cmlen - sizeof(*cm), efault_end);
+ unsafe_put_user(type, &cm->cmsg_type, efault_end);
user_write_access_end();
} else {
struct cmsghdr *cm = msg->msg_control;

such that we only copy cm->cmsg_type after we transfered the data.

2023-04-17 17:53:45

by Eric Dumazet

[permalink] [raw]

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

On Mon, Apr 17, 2023 at 7:16 PM Christian Brauner <[email protected]> wrote:
>
> On Mon, Apr 17, 2023 at 06:01:16PM +0200, Aleksandr Mikhalitsyn wrote:
> > On Mon, Apr 17, 2023 at 5:18 PM Christian Brauner <[email protected]> wrote:
> > >
> > > 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" <[email protected]>
> > > > Cc: Eric Dumazet <[email protected]>
> > > > Cc: Jakub Kicinski <[email protected]>
> > > > Cc: Paolo Abeni <[email protected]>
> > > > Cc: Leon Romanovsky <[email protected]>
> > > > Cc: David Ahern <[email protected]>
> > > > Cc: Arnd Bergmann <[email protected]>
> > > > Cc: Kees Cook <[email protected]>
> > > > Cc: Christian Brauner <[email protected]>
> > > > Cc: Kuniyuki Iwashima <[email protected]>
> > > > Cc: Lennart Poettering <[email protected]>
> > > > Cc: Luca Boccassi <[email protected]>
> > > > Cc: [email protected]
> > > > Cc: [email protected]
> > > > Cc: [email protected]
> > > > Tested-by: Luca Boccassi <[email protected]>
> > > > Reviewed-by: Kuniyuki Iwashima <[email protected]>
> > > > Signed-off-by: Alexander Mikhalitsyn <[email protected]>
> > > > ---
> > > > 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?
> >
> > Hi, Christian!
> >
> > For SCM_CREDENTIALS we mark it too. Inside the put_cmsg function:
> > https://github.com/torvalds/linux/blob/6a8f57ae2eb07ab39a6f0ccad60c760743051026/net/core/scm.c#L225
> >
> > The reason why I'm open-coding these checks is that I want to know
> > that the message
> > doesn't fit into the userspace buffer before doing pidfd_prepare and
> > other stuff and because
> > put_cmsg is not returning an error when message doesn't fit in the
> > userspace buffer and
> > we won't be able to properly do pidfd cleanup (put struct pid and fd index).
> >
> > >
> > > > + }
> > > > +
> > > > + 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.
> >
> > If pidfd_prepare fails then userspace will receive SCM_PIDFD message
> > with negative pidfd value.
>
> So we discussed this a bit offline and I think there's still an issue.
> If put_cmsg() fails
>
> if (msg->msg_control_is_user) {
> struct cmsghdr __user *cm = msg->msg_control_user;
>
> check_object_size(data, cmlen - sizeof(*cm), true);
>
> if (!user_write_access_begin(cm, cmlen))
> goto efault;
>
> // This succeeds so cm->cmsg_len == sizeof(int)
> unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);
>
> // This succeeds so cm->cmsg_level == SOL_SOCKET
> unsafe_put_user(level, &cm->cmsg_level, efault_end);
>
> // This succeeds so cm->cmsg_type == SCM_PIDFD
> unsafe_put_user(type, &cm->cmsg_type, efault_end);
>
> // This fails and leaves all bits set to 0
> unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
> cmlen - sizeof(*cm), efault_end);
> user_write_access_end();
>
> so now we hit
>
> if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {
> if (pidfd_file) {
> put_unused_fd(pidfd);
> fput(pidfd_file);
> }
>
> return;
> }
>
> and return early. Afaict, userspace would now receive:
>
> if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
> cmsg->cmsg_level == SOL_SOCKET &&
> cmsg->cmsg_type == SCM_PIDFD) {
> memcpy(&pidfd, CMSG_DATA(cmsg), sizeof(int));
>
> // pidfd is now 0 which is a valid fd number
> // it'll likely refer to /dev/stdin or whatever and so
> // will fail or, worst case, 0 refers to another pidfd :)
> pidfd_send_signal(pidfd, SIGKILL);
>
> so we need to address this. So one way I think that would solve this is:
>
> diff --git a/net/core/scm.c b/net/core/scm.c
> index 3cd7dd377e53..d1f4cd135c5a 100644
> --- a/net/core/scm.c
> +++ b/net/core/scm.c
> @@ -236,9 +236,9 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
>
> unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);
> unsafe_put_user(level, &cm->cmsg_level, efault_end);
> - unsafe_put_user(type, &cm->cmsg_type, efault_end);
> unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
> cmlen - sizeof(*cm), efault_end);
> + unsafe_put_user(type, &cm->cmsg_type, efault_end);
> user_write_access_end();
> } else {
> struct cmsghdr *cm = msg->msg_control;
>
> such that we only copy cm->cmsg_type after we transfered the data.

This looks wrong to me.

if put_cmsg() returns -EFAULT, then msg->msg_control and
msg->msg_controllen were not changed.

So the user application should not attempt to read this part of the
control buffer, this could contain garbage.

2023-04-18 08:33:56

by Christian Brauner

[permalink] [raw]

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

On Mon, Apr 17, 2023 at 07:43:19PM +0200, Eric Dumazet wrote:
> On Mon, Apr 17, 2023 at 7:16 PM Christian Brauner <[email protected]> wrote:
> >
> > On Mon, Apr 17, 2023 at 06:01:16PM +0200, Aleksandr Mikhalitsyn wrote:
> > > On Mon, Apr 17, 2023 at 5:18 PM Christian Brauner <[email protected]> wrote:
> > > >
> > > > 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" <[email protected]>
> > > > > Cc: Eric Dumazet <[email protected]>
> > > > > Cc: Jakub Kicinski <[email protected]>
> > > > > Cc: Paolo Abeni <[email protected]>
> > > > > Cc: Leon Romanovsky <[email protected]>
> > > > > Cc: David Ahern <[email protected]>
> > > > > Cc: Arnd Bergmann <[email protected]>
> > > > > Cc: Kees Cook <[email protected]>
> > > > > Cc: Christian Brauner <[email protected]>
> > > > > Cc: Kuniyuki Iwashima <[email protected]>
> > > > > Cc: Lennart Poettering <[email protected]>
> > > > > Cc: Luca Boccassi <[email protected]>
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Tested-by: Luca Boccassi <[email protected]>
> > > > > Reviewed-by: Kuniyuki Iwashima <[email protected]>
> > > > > Signed-off-by: Alexander Mikhalitsyn <[email protected]>
> > > > > ---
> > > > > 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?
> > >
> > > Hi, Christian!
> > >
> > > For SCM_CREDENTIALS we mark it too. Inside the put_cmsg function:
> > > https://github.com/torvalds/linux/blob/6a8f57ae2eb07ab39a6f0ccad60c760743051026/net/core/scm.c#L225
> > >
> > > The reason why I'm open-coding these checks is that I want to know
> > > that the message
> > > doesn't fit into the userspace buffer before doing pidfd_prepare and
> > > other stuff and because
> > > put_cmsg is not returning an error when message doesn't fit in the
> > > userspace buffer and
> > > we won't be able to properly do pidfd cleanup (put struct pid and fd index).
> > >
> > > >
> > > > > + }
> > > > > +
> > > > > + 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.
> > >
> > > If pidfd_prepare fails then userspace will receive SCM_PIDFD message
> > > with negative pidfd value.
> >
> > So we discussed this a bit offline and I think there's still an issue.
> > If put_cmsg() fails
> >
> > if (msg->msg_control_is_user) {
> > struct cmsghdr __user *cm = msg->msg_control_user;
> >
> > check_object_size(data, cmlen - sizeof(*cm), true);
> >
> > if (!user_write_access_begin(cm, cmlen))
> > goto efault;
> >
> > // This succeeds so cm->cmsg_len == sizeof(int)
> > unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);
> >
> > // This succeeds so cm->cmsg_level == SOL_SOCKET
> > unsafe_put_user(level, &cm->cmsg_level, efault_end);
> >
> > // This succeeds so cm->cmsg_type == SCM_PIDFD
> > unsafe_put_user(type, &cm->cmsg_type, efault_end);
> >
> > // This fails and leaves all bits set to 0
> > unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
> > cmlen - sizeof(*cm), efault_end);
> > user_write_access_end();
> >
> > so now we hit
> >
> > if (put_cmsg(msg, SOL_SOCKET, SCM_PIDFD, sizeof(int), &pidfd)) {
> > if (pidfd_file) {
> > put_unused_fd(pidfd);
> > fput(pidfd_file);
> > }
> >
> > return;
> > }
> >
> > and return early. Afaict, userspace would now receive:
> >
> > if (cmsg && cmsg->cmsg_len == CMSG_LEN(sizeof(int)) &&
> > cmsg->cmsg_level == SOL_SOCKET &&
> > cmsg->cmsg_type == SCM_PIDFD) {
> > memcpy(&pidfd, CMSG_DATA(cmsg), sizeof(int));
> >
> > // pidfd is now 0 which is a valid fd number
> > // it'll likely refer to /dev/stdin or whatever and so
> > // will fail or, worst case, 0 refers to another pidfd :)
> > pidfd_send_signal(pidfd, SIGKILL);
> >
> > so we need to address this. So one way I think that would solve this is:
> >
> > diff --git a/net/core/scm.c b/net/core/scm.c
> > index 3cd7dd377e53..d1f4cd135c5a 100644
> > --- a/net/core/scm.c
> > +++ b/net/core/scm.c
> > @@ -236,9 +236,9 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
> >
> > unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);
> > unsafe_put_user(level, &cm->cmsg_level, efault_end);
> > - unsafe_put_user(type, &cm->cmsg_type, efault_end);
> > unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
> > cmlen - sizeof(*cm), efault_end);
> > + unsafe_put_user(type, &cm->cmsg_type, efault_end);
> > user_write_access_end();
> > } else {
> > struct cmsghdr *cm = msg->msg_control;
> >
> > such that we only copy cm->cmsg_type after we transfered the data.
>
> This looks wrong to me.
>
> if put_cmsg() returns -EFAULT, then msg->msg_control and
> msg->msg_controllen were not changed.
>
> So the user application should not attempt to read this part of the
> control buffer, this could contain garbage.

Thanks for the review, Eric. That's reassuring.

I've done a bit of container related networking before but I'm fumbling
my way through the reviews here. So any additional reviews here would be
very helpful.

2023-04-18 13:13:36

by Christian Brauner

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