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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Heiko Carstens <hca@linux.ibm.com>
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" <davem@davemloft.net>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Paolo Abeni <pabeni@redhat.com>
> > Cc: Leon Romanovsky <leon@kernel.org>
> > Cc: David Ahern <dsahern@kernel.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
> > Cc: Lennart Poettering <mzxreary@0pointer.de>
> > Cc: Luca Boccassi <bluca@debian.org>
> > Cc: linux-kernel@vger.kernel.org
> > Cc: netdev@vger.kernel.org
> > Cc: linux-arch@vger.kernel.org
> > Tested-by: Luca Boccassi <bluca@debian.org>
> > Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > Reviewed-by: Christian Brauner <brauner@kernel.org>
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.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.

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<---