[PATCH 0/3] Clean up and fix SCM_CREDENTIALS code

[PATCH 0/3] Clean up and fix SCM_CREDENTIALS code

[Andy Lutomirski]

The code is incomprehensible, slow, and buggy.

Andy Lutomirski (3):
  net: Clean up SCM_CREDENTIALS code
  netlink: Remove an unused pointer in netlink_skb_parms
  net: Remove sock_iocb.scm

 include/linux/netlink.h  |  1 -
 include/net/af_unix.h    |  6 ++--
 include/net/scm.h        | 83 ++++++++++++++++++++++++++++++++----------------
 include/net/sock.h       |  1 -
 net/core/scm.c           | 49 ++++++++++------------------
 net/netlink/af_netlink.c | 31 ++++++++----------
 net/socket.c             |  2 --
 net/unix/af_unix.c       | 83 +++++++++++++++++-------------------------------
 8 files changed, 118 insertions(+), 138 deletions(-)

-- 
1.8.1.4

[PATCH 1/3] net: Clean up SCM_CREDENTIALS code

[Andy Lutomirski]

I was curious whether the uids, gids, and pids passed around worked
correctly in the presence of multiple namespaces.  I gave up trying
to figure it out: there are two copies of the pid (one of which has
type u32, which is odd), a struct cred * (!), and a separate kuid
and kgid.  IOW, all of the relevant data is stored twice, and it's
unclear which copy is used when.

I also wondered what prevented a SO_CREDENTIALS message from being
recieved when the credentials weren't filled out.  Answer: not very
much (and there have been serious security bugs here in the past).

So just rewrite the thing to store a pid_t relative to the init pid
ns, a kuid, and a kgid, and to explicitly track whether the data is
filled out.

I haven't played with the secid code.  I have no idea whether it has
similar problems.

I haven't benchmarked this, but it should be a respectable speedup
in the cases where the credentials are in use.

Signed-off-by: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
---

Before, the program below printed this:

$ passcred
My pid = 19873
No SO_PASSCRED: uid=65534 gid=65534 pid=0  [this is a bug]
SO_PASSCRED: uid=1000 gid=1000 pid=19873
SO_PASSCRED, forked to pid 19874: uid=1000 gid=1000 pid=19874

# passcred
My pid = 19886
No SO_PASSCRED: uid=65534 gid=65534 pid=0
SO_PASSCRED: uid=0 gid=0 pid=19886
SO_PASSCRED, forked to pid 19887: uid=0 gid=0 pid=19887


After:

# passcred
My pid = 83
No creds received
SO_PASSCRED: uid=0 gid=0 pid=83
SO_PASSCRED, forked to pid 84: uid=0 gid=0 pid=0


#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <err.h>

static void send_str(int fd, const char *msg)
{
	if (send(fd, msg, strlen(msg)+1, 0) < 0)
		err(1, "send");
}

int main()
{
	int fds[2];
	if (socketpair(AF_UNIX, SOCK_DGRAM, 0, fds))
		err(1, "socketpair");

	send_str(fds[1], "No SO_PASSCRED");

	int one = 1;
	if (setsockopt(fds[0], SOL_SOCKET, SO_PASSCRED, &one, sizeof(one)) != 0)
		err(1, "SO_PASSCRED");

	send_str(fds[1], "SO_PASSCRED");

	if (fork() == 0) {
		char msg[1024];
		sprintf(msg, "SO_PASSCRED, forked to pid %ld", (long)getpid());
		send_str(fds[1], msg);
		return 0;
	}

	int status;
	wait(&status);
	printf("My pid = %ld\n", getpid());

	for (int i = 0; i < 3; i++) {
		char buf[1024];
		char cbuf[CMSG_SPACE(sizeof(struct ucred))];
		struct iovec iov;
		iov.iov_base = &buf;
		iov.iov_len = sizeof(buf);
		struct msghdr hdr;
		memset(&hdr, 0, sizeof(hdr));
		hdr.msg_iov = &iov;
		hdr.msg_iovlen = 1;
		hdr.msg_control = cbuf;
		hdr.msg_controllen = sizeof(cbuf);
		ssize_t bytes = recvmsg(fds[0], &hdr, 0);
		if (bytes < 0)
			err(1, "recvmsg");

		if (hdr.msg_flags & (MSG_TRUNC | MSG_CTRUNC))
			errx(1, "truncated");

		struct ucred cred;
		bool ok = false;

		for (struct cmsghdr *cmsg = CMSG_FIRSTHDR(&hdr); cmsg; cmsg = CMSG_NXTHDR(&hdr, cmsg)) {
			if (cmsg->cmsg_level == SOL_SOCKET &&
			    cmsg->cmsg_type == SCM_CREDENTIALS) {
				cred = *((struct ucred *)CMSG_DATA(cmsg));
				ok = true;
				break;
			}
		}
		if (!ok) {
			printf("No creds received\n");
		} else {
			printf("%s: uid=%ld gid=%ld pid=%ld\n",
			       buf, (long)cred.uid, (long)cred.gid, (long)cred.pid);
		}
	}

	return 0;
}


 include/net/af_unix.h    |  6 ++--
 include/net/scm.h        | 83 ++++++++++++++++++++++++++++++++----------------
 net/core/scm.c           | 49 ++++++++++------------------
 net/netlink/af_netlink.c | 14 +++++---
 net/unix/af_unix.c       | 35 +++++++++-----------
 5 files changed, 99 insertions(+), 88 deletions(-)

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 0a996a3..7874f3e 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -27,12 +27,12 @@ struct unix_address {
 	struct sockaddr_un name[0];
 };
 
+/* This structure is identical to struct scm_cookie. */
 struct unix_skb_parms {
-	struct pid		*pid;		/* Skb credentials	*/
-	const struct cred	*cred;
+	struct scm_creds	creds;
 	struct scm_fp_list	*fp;		/* Passed files		*/
 #ifdef CONFIG_SECURITY_NETWORK
-	u32			secid;		/* Security ID		*/
+	u32			secid;		/* Passed security ID 	*/
 #endif
 };
 
diff --git a/include/net/scm.h b/include/net/scm.h
index 975cca0..f6f0626 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h
@@ -13,9 +13,24 @@
 #define SCM_MAX_FD	253
 
 struct scm_creds {
-	u32	pid;
-	kuid_t	uid;
-	kgid_t	gid;
+	bool has_creds;
+
+	/*
+	 * Keeping reference counts (as to a struct pid *) in here is
+	 * annoying -- things like skb_set_owner_[rw] and skb_clone assume
+	 * that it's ok to memcpy skb->cb around.
+	 *
+	 * Fortunately (?) anything that uses the pid field in SCM_CREDENTIALS
+	 * is fundamentally racy, since the networking code certainly isn't
+	 * going to keep a reference alive *after* recvmsg.  So let's embrace
+	 * the race condition at the cost of an extra hash lookup on receive.
+	 *
+	 * (There's an added benefit here: this approach doesn't write to
+	 * any shared cachelines.)
+	 */
+	pid_t		init_ns_pid;
+	kuid_t		uid;
+	kgid_t		gid;
 };
 
 struct scm_fp_list {
@@ -25,15 +40,15 @@ struct scm_fp_list {
 };
 
 struct scm_cookie {
-	struct pid		*pid;		/* Skb credentials */
-	const struct cred	*cred;
+	struct scm_creds	creds;
 	struct scm_fp_list	*fp;		/* Passed files		*/
-	struct scm_creds	creds;		/* Skb credentials	*/
 #ifdef CONFIG_SECURITY_NETWORK
 	u32			secid;		/* Passed security ID 	*/
 #endif
 };
 
+#define SCM_COOKIE_INIT {}  /* All zeros is good. */
+
 extern void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm);
 extern void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm);
 extern int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm);
@@ -50,39 +65,47 @@ static __inline__ void unix_get_peersec_dgram(struct socket *sock, struct scm_co
 { }
 #endif /* CONFIG_SECURITY_NETWORK */
 
-static __inline__ void scm_set_cred(struct scm_cookie *scm,
-				    struct pid *pid, const struct cred *cred)
+static __inline__ bool scm_creds_equal(const struct scm_creds *a,
+                                       const struct scm_creds *b)
 {
-	scm->pid  = get_pid(pid);
-	scm->cred = cred ? get_cred(cred) : NULL;
-	scm->creds.pid = pid_vnr(pid);
-	scm->creds.uid = cred ? cred->euid : INVALID_UID;
-	scm->creds.gid = cred ? cred->egid : INVALID_GID;
+	if (a->has_creds)
+		return a->init_ns_pid == b->init_ns_pid &&
+			uid_eq(a->uid, b->uid) && gid_eq(a->gid, b->gid);
+	else
+		return !b->has_creds;
 }
 
-static __inline__ void scm_destroy_cred(struct scm_cookie *scm)
+static __inline__ void scm_creds_from_current(struct scm_creds *creds)
 {
-	put_pid(scm->pid);
-	scm->pid  = NULL;
+	const struct cred *cred = get_current_cred();
+	creds->has_creds = true;
+	creds->init_ns_pid = task_tgid_nr(current);
+	creds->uid = cred->uid;
+	creds->gid = cred->gid;
+}
 
-	if (scm->cred)
-		put_cred(scm->cred);
-	scm->cred = NULL;
+static __inline__ void scm_creds_from_kernel(struct scm_creds *creds)
+{
+	creds->has_creds = true;
+	creds->init_ns_pid = 0;
+	creds->uid = GLOBAL_ROOT_UID;
+	creds->gid = GLOBAL_ROOT_GID;
+}
+
+static __inline__ void scm_creds_wipe(struct scm_creds *creds)
+{
+	creds->has_creds = false;
 }
 
 static __inline__ void scm_destroy(struct scm_cookie *scm)
 {
-	scm_destroy_cred(scm);
 	if (scm->fp)
 		__scm_destroy(scm);
 }
 
 static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
-			       struct scm_cookie *scm, bool forcecreds)
+			       struct scm_cookie *scm)
 {
-	memset(scm, 0, sizeof(*scm));
-	if (forcecreds)
-		scm_set_cred(scm, task_tgid(current), current_cred());
 	unix_get_peersec_dgram(sock, scm);
 	if (msg->msg_controllen <= 0)
 		return 0;
@@ -120,18 +143,22 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
 		return;
 	}
 
-	if (test_bit(SOCK_PASSCRED, &sock->flags)) {
+	if (test_bit(SOCK_PASSCRED, &sock->flags) && scm->creds.has_creds) {
 		struct user_namespace *current_ns = current_user_ns();
 		struct ucred ucreds = {
-			.pid = scm->creds.pid,
 			.uid = from_kuid_munged(current_ns, scm->creds.uid),
 			.gid = from_kgid_munged(current_ns, scm->creds.gid),
 		};
+
+		rcu_read_lock();
+		/* On error, this will result in pid 0. */
+		ucreds.pid = pid_vnr(find_pid_ns(scm->creds.init_ns_pid,
+						 &init_pid_ns));
+		rcu_read_unlock();
+
 		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(ucreds), &ucreds);
 	}
 
-	scm_destroy_cred(scm);
-
 	scm_passec(sock, msg, scm);
 
 	if (!scm->fp)
diff --git a/net/core/scm.c b/net/core/scm.c
index 905dcc6..7dd7534 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -157,8 +157,7 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
 		case SCM_CREDENTIALS:
 		{
 			struct ucred creds;
-			kuid_t uid;
-			kgid_t gid;
+			struct pid *pid;
 			if (cmsg->cmsg_len != CMSG_LEN(sizeof(struct ucred)))
 				goto error;
 			memcpy(&creds, CMSG_DATA(cmsg), sizeof(struct ucred));
@@ -166,41 +165,25 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
 			if (err)
 				goto error;
 
-			p->creds.pid = creds.pid;
-			if (!p->pid || pid_vnr(p->pid) != creds.pid) {
-				struct pid *pid;
-				err = -ESRCH;
-				pid = find_get_pid(creds.pid);
-				if (!pid)
-					goto error;
-				put_pid(p->pid);
-				p->pid = pid;
-			}
-
 			err = -EINVAL;
-			uid = make_kuid(current_user_ns(), creds.uid);
-			gid = make_kgid(current_user_ns(), creds.gid);
-			if (!uid_valid(uid) || !gid_valid(gid))
+			p->creds.uid = make_kuid(current_user_ns(), creds.uid);
+			p->creds.gid = make_kgid(current_user_ns(), creds.gid);
+			if (!uid_valid(p->creds.uid) ||
+			    !gid_valid(p->creds.gid))
 				goto error;
 
-			p->creds.uid = uid;
-			p->creds.gid = gid;
-
-			if (!p->cred ||
-			    !uid_eq(p->cred->euid, uid) ||
-			    !gid_eq(p->cred->egid, gid)) {
-				struct cred *cred;
-				err = -ENOMEM;
-				cred = prepare_creds();
-				if (!cred)
-					goto error;
-
-				cred->uid = cred->euid = uid;
-				cred->gid = cred->egid = gid;
-				if (p->cred)
-					put_cred(p->cred);
-				p->cred = cred;
+			rcu_read_lock();
+			pid = find_vpid(creds.pid);
+			if (!pid) {
+				rcu_read_unlock();
+				err = -ESRCH;
+				goto error;
 			}
+			p->creds.init_ns_pid = pid_nr(pid);
+			rcu_read_unlock();
+
+			p->creds.has_creds = true;
+
 			break;
 		}
 		default:
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 1e3fd5b..8245f61 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -936,7 +936,6 @@ static int netlink_unicast_kernel(struct sock *sk, struct sk_buff *skb,
 	if (nlk->netlink_rcv != NULL) {
 		ret = skb->len;
 		skb_set_owner_r(skb, sk);
-		NETLINK_CB(skb).ssk = ssk;
 		nlk->netlink_rcv(skb);
 		consume_skb(skb);
 	} else {
@@ -1368,7 +1367,7 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	u32 dst_group;
 	struct sk_buff *skb;
 	int err;
-	struct scm_cookie scm;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 
 	if (msg->msg_flags&MSG_OOB)
 		return -EOPNOTSUPP;
@@ -1376,7 +1375,8 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	if (NULL == siocb->scm)
 		siocb->scm = &scm;
 
-	err = scm_send(sock, msg, siocb->scm, true);
+	scm_creds_from_current(&siocb->scm->creds);
+	err = scm_send(sock, msg, siocb->scm);
 	if (err < 0)
 		return err;
 
@@ -1411,7 +1411,9 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 
 	NETLINK_CB(skb).portid	= nlk->portid;
 	NETLINK_CB(skb).dst_group = dst_group;
-	NETLINK_CB(skb).creds	= siocb->scm->creds;
+
+	/* This is mandatory. See netlink_recvmsg. */
+	NETLINK_CB(skb).creds = siocb->scm->creds;
 
 	err = -EFAULT;
 	if (memcpy_fromiovec(skb_put(skb, len), msg->msg_iov, len)) {
@@ -1504,7 +1506,11 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 		memset(&scm, 0, sizeof(scm));
 		siocb->scm = &scm;
 	}
+	/* skbs without creds are from the kernel. */
 	siocb->scm->creds = *NETLINK_CREDS(skb);
+	if (!siocb->scm->creds.has_creds)
+		scm_creds_from_kernel(&siocb->scm->creds);
+
 	if (flags & MSG_TRUNC)
 		copied = data_skb->len;
 
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 51be64f..0881739 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1338,10 +1338,8 @@ static void unix_detach_fds(struct scm_cookie *scm, struct sk_buff *skb)
 
 static void unix_destruct_scm(struct sk_buff *skb)
 {
-	struct scm_cookie scm;
-	memset(&scm, 0, sizeof(scm));
-	scm.pid  = UNIXCB(skb).pid;
-	scm.cred = UNIXCB(skb).cred;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
+	scm.creds = UNIXCB(skb).creds;
 	if (UNIXCB(skb).fp)
 		unix_detach_fds(&scm, skb);
 
@@ -1391,9 +1389,7 @@ static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool sen
 {
 	int err = 0;
 
-	UNIXCB(skb).pid  = get_pid(scm->pid);
-	if (scm->cred)
-		UNIXCB(skb).cred = get_cred(scm->cred);
+	UNIXCB(skb).creds = scm->creds;
 	UNIXCB(skb).fp = NULL;
 	if (scm->fp && send_fds)
 		err = unix_attach_fds(scm, skb);
@@ -1410,13 +1406,12 @@ static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool sen
 static void maybe_add_creds(struct sk_buff *skb, const struct socket *sock,
 			    const struct sock *other)
 {
-	if (UNIXCB(skb).cred)
+	if (UNIXCB(skb).creds.has_creds)
 		return;
 	if (test_bit(SOCK_PASSCRED, &sock->flags) ||
 	    !other->sk_socket ||
 	    test_bit(SOCK_PASSCRED, &other->sk_socket->flags)) {
-		UNIXCB(skb).pid  = get_pid(task_tgid(current));
-		UNIXCB(skb).cred = get_current_cred();
+		scm_creds_from_current(&UNIXCB(skb).creds);
 	}
 }
 
@@ -1438,14 +1433,14 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	unsigned int hash;
 	struct sk_buff *skb;
 	long timeo;
-	struct scm_cookie tmp_scm;
+	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
 	int max_level;
 	int data_len = 0;
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
 	wait_for_unix_gc();
-	err = scm_send(sock, msg, siocb->scm, false);
+	err = scm_send(sock, msg, siocb->scm);
 	if (err < 0)
 		return err;
 
@@ -1607,14 +1602,14 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	int err, size;
 	struct sk_buff *skb;
 	int sent = 0;
-	struct scm_cookie tmp_scm;
+	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
 	bool fds_sent = false;
 	int max_level;
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
 	wait_for_unix_gc();
-	err = scm_send(sock, msg, siocb->scm, false);
+	err = scm_send(sock, msg, siocb->scm);
 	if (err < 0)
 		return err;
 
@@ -1765,7 +1760,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 			      int flags)
 {
 	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
-	struct scm_cookie tmp_scm;
+	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
 	int noblock = flags & MSG_DONTWAIT;
@@ -1820,7 +1815,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 		siocb->scm = &tmp_scm;
 		memset(&tmp_scm, 0, sizeof(tmp_scm));
 	}
-	scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
+	siocb->scm->creds = UNIXCB(skb).creds;
 	unix_set_secdata(siocb->scm, skb);
 
 	if (!(flags & MSG_PEEK)) {
@@ -1898,7 +1893,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 			       int flags)
 {
 	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
-	struct scm_cookie tmp_scm;
+	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
 	struct sockaddr_un *sunaddr = msg->msg_name;
@@ -1991,12 +1986,12 @@ again:
 
 		if (check_creds) {
 			/* Never glue messages from different writers */
-			if ((UNIXCB(skb).pid  != siocb->scm->pid) ||
-			    (UNIXCB(skb).cred != siocb->scm->cred))
+			if (!scm_creds_equal(&UNIXCB(skb).creds,
+			                     &siocb->scm->creds));
 				break;
 		} else {
 			/* Copy credentials */
-			scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
+			siocb->scm->creds = UNIXCB(skb).creds;
 			check_creds = 1;
 		}
 
-- 
1.8.1.4

Re: [PATCH 1/3] net: Clean up SCM_CREDENTIALS code

[James Morris]

(adding David Howells)


On Wed, 20 Mar 2013, Andy Lutomirski wrote:

> I was curious whether the uids, gids, and pids passed around worked
> correctly in the presence of multiple namespaces.  I gave up trying
> to figure it out: there are two copies of the pid (one of which has
> type u32, which is odd), a struct cred * (!), and a separate kuid
> and kgid.  IOW, all of the relevant data is stored twice, and it's
> unclear which copy is used when.
> 
> I also wondered what prevented a SO_CREDENTIALS message from being
> recieved when the credentials weren't filled out.  Answer: not very
> much (and there have been serious security bugs here in the past).
> 
> So just rewrite the thing to store a pid_t relative to the init pid
> ns, a kuid, and a kgid, and to explicitly track whether the data is
> filled out.
> 
> I haven't played with the secid code.  I have no idea whether it has
> similar problems.
> 
> I haven't benchmarked this, but it should be a respectable speedup
> in the cases where the credentials are in use.
> 
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
> 
> Before, the program below printed this:
> 
> $ passcred
> My pid = 19873
> No SO_PASSCRED: uid=65534 gid=65534 pid=0  [this is a bug]
> SO_PASSCRED: uid=1000 gid=1000 pid=19873
> SO_PASSCRED, forked to pid 19874: uid=1000 gid=1000 pid=19874
> 
> # passcred
> My pid = 19886
> No SO_PASSCRED: uid=65534 gid=65534 pid=0
> SO_PASSCRED: uid=0 gid=0 pid=19886
> SO_PASSCRED, forked to pid 19887: uid=0 gid=0 pid=19887
> 
> 
> After:
> 
> # passcred
> My pid = 83
> No creds received
> SO_PASSCRED: uid=0 gid=0 pid=83
> SO_PASSCRED, forked to pid 84: uid=0 gid=0 pid=0
> 
> 
> #define _GNU_SOURCE
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <sys/socket.h>
> #include <stdlib.h>
> #include <stdint.h>
> #include <stdbool.h>
> #include <stdio.h>
> #include <string.h>
> #include <unistd.h>
> #include <err.h>
> 
> static void send_str(int fd, const char *msg)
> {
> 	if (send(fd, msg, strlen(msg)+1, 0) < 0)
> 		err(1, "send");
> }
> 
> int main()
> {
> 	int fds[2];
> 	if (socketpair(AF_UNIX, SOCK_DGRAM, 0, fds))
> 		err(1, "socketpair");
> 
> 	send_str(fds[1], "No SO_PASSCRED");
> 
> 	int one = 1;
> 	if (setsockopt(fds[0], SOL_SOCKET, SO_PASSCRED, &one, sizeof(one)) != 0)
> 		err(1, "SO_PASSCRED");
> 
> 	send_str(fds[1], "SO_PASSCRED");
> 
> 	if (fork() == 0) {
> 		char msg[1024];
> 		sprintf(msg, "SO_PASSCRED, forked to pid %ld", (long)getpid());
> 		send_str(fds[1], msg);
> 		return 0;
> 	}
> 
> 	int status;
> 	wait(&status);
> 	printf("My pid = %ld\n", getpid());
> 
> 	for (int i = 0; i < 3; i++) {
> 		char buf[1024];
> 		char cbuf[CMSG_SPACE(sizeof(struct ucred))];
> 		struct iovec iov;
> 		iov.iov_base = &buf;
> 		iov.iov_len = sizeof(buf);
> 		struct msghdr hdr;
> 		memset(&hdr, 0, sizeof(hdr));
> 		hdr.msg_iov = &iov;
> 		hdr.msg_iovlen = 1;
> 		hdr.msg_control = cbuf;
> 		hdr.msg_controllen = sizeof(cbuf);
> 		ssize_t bytes = recvmsg(fds[0], &hdr, 0);
> 		if (bytes < 0)
> 			err(1, "recvmsg");
> 
> 		if (hdr.msg_flags & (MSG_TRUNC | MSG_CTRUNC))
> 			errx(1, "truncated");
> 
> 		struct ucred cred;
> 		bool ok = false;
> 
> 		for (struct cmsghdr *cmsg = CMSG_FIRSTHDR(&hdr); cmsg; cmsg = CMSG_NXTHDR(&hdr, cmsg)) {
> 			if (cmsg->cmsg_level == SOL_SOCKET &&
> 			    cmsg->cmsg_type == SCM_CREDENTIALS) {
> 				cred = *((struct ucred *)CMSG_DATA(cmsg));
> 				ok = true;
> 				break;
> 			}
> 		}
> 		if (!ok) {
> 			printf("No creds received\n");
> 		} else {
> 			printf("%s: uid=%ld gid=%ld pid=%ld\n",
> 			       buf, (long)cred.uid, (long)cred.gid, (long)cred.pid);
> 		}
> 	}
> 
> 	return 0;
> }
> 
> 
>  include/net/af_unix.h    |  6 ++--
>  include/net/scm.h        | 83 ++++++++++++++++++++++++++++++++----------------
>  net/core/scm.c           | 49 ++++++++++------------------
>  net/netlink/af_netlink.c | 14 +++++---
>  net/unix/af_unix.c       | 35 +++++++++-----------
>  5 files changed, 99 insertions(+), 88 deletions(-)
> 
> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
> index 0a996a3..7874f3e 100644
> --- a/include/net/af_unix.h
> +++ b/include/net/af_unix.h
> @@ -27,12 +27,12 @@ struct unix_address {
>  	struct sockaddr_un name[0];
>  };
>  
> +/* This structure is identical to struct scm_cookie. */
>  struct unix_skb_parms {
> -	struct pid		*pid;		/* Skb credentials	*/
> -	const struct cred	*cred;
> +	struct scm_creds	creds;
>  	struct scm_fp_list	*fp;		/* Passed files		*/
>  #ifdef CONFIG_SECURITY_NETWORK
> -	u32			secid;		/* Security ID		*/
> +	u32			secid;		/* Passed security ID 	*/
>  #endif
>  };
>  
> diff --git a/include/net/scm.h b/include/net/scm.h
> index 975cca0..f6f0626 100644
> --- a/include/net/scm.h
> +++ b/include/net/scm.h
> @@ -13,9 +13,24 @@
>  #define SCM_MAX_FD	253
>  
>  struct scm_creds {
> -	u32	pid;
> -	kuid_t	uid;
> -	kgid_t	gid;
> +	bool has_creds;
> +
> +	/*
> +	 * Keeping reference counts (as to a struct pid *) in here is
> +	 * annoying -- things like skb_set_owner_[rw] and skb_clone assume
> +	 * that it's ok to memcpy skb->cb around.
> +	 *
> +	 * Fortunately (?) anything that uses the pid field in SCM_CREDENTIALS
> +	 * is fundamentally racy, since the networking code certainly isn't
> +	 * going to keep a reference alive *after* recvmsg.  So let's embrace
> +	 * the race condition at the cost of an extra hash lookup on receive.
> +	 *
> +	 * (There's an added benefit here: this approach doesn't write to
> +	 * any shared cachelines.)
> +	 */
> +	pid_t		init_ns_pid;
> +	kuid_t		uid;
> +	kgid_t		gid;
>  };
>  
>  struct scm_fp_list {
> @@ -25,15 +40,15 @@ struct scm_fp_list {
>  };
>  
>  struct scm_cookie {
> -	struct pid		*pid;		/* Skb credentials */
> -	const struct cred	*cred;
> +	struct scm_creds	creds;
>  	struct scm_fp_list	*fp;		/* Passed files		*/
> -	struct scm_creds	creds;		/* Skb credentials	*/
>  #ifdef CONFIG_SECURITY_NETWORK
>  	u32			secid;		/* Passed security ID 	*/
>  #endif
>  };
>  
> +#define SCM_COOKIE_INIT {}  /* All zeros is good. */
> +
>  extern void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm);
>  extern void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm);
>  extern int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm);
> @@ -50,39 +65,47 @@ static __inline__ void unix_get_peersec_dgram(struct socket *sock, struct scm_co
>  { }
>  #endif /* CONFIG_SECURITY_NETWORK */
>  
> -static __inline__ void scm_set_cred(struct scm_cookie *scm,
> -				    struct pid *pid, const struct cred *cred)
> +static __inline__ bool scm_creds_equal(const struct scm_creds *a,
> +                                       const struct scm_creds *b)
>  {
> -	scm->pid  = get_pid(pid);
> -	scm->cred = cred ? get_cred(cred) : NULL;
> -	scm->creds.pid = pid_vnr(pid);
> -	scm->creds.uid = cred ? cred->euid : INVALID_UID;
> -	scm->creds.gid = cred ? cred->egid : INVALID_GID;
> +	if (a->has_creds)
> +		return a->init_ns_pid == b->init_ns_pid &&
> +			uid_eq(a->uid, b->uid) && gid_eq(a->gid, b->gid);
> +	else
> +		return !b->has_creds;
>  }
>  
> -static __inline__ void scm_destroy_cred(struct scm_cookie *scm)
> +static __inline__ void scm_creds_from_current(struct scm_creds *creds)
>  {
> -	put_pid(scm->pid);
> -	scm->pid  = NULL;
> +	const struct cred *cred = get_current_cred();
> +	creds->has_creds = true;
> +	creds->init_ns_pid = task_tgid_nr(current);
> +	creds->uid = cred->uid;
> +	creds->gid = cred->gid;
> +}
>  
> -	if (scm->cred)
> -		put_cred(scm->cred);
> -	scm->cred = NULL;
> +static __inline__ void scm_creds_from_kernel(struct scm_creds *creds)
> +{
> +	creds->has_creds = true;
> +	creds->init_ns_pid = 0;
> +	creds->uid = GLOBAL_ROOT_UID;
> +	creds->gid = GLOBAL_ROOT_GID;
> +}
> +
> +static __inline__ void scm_creds_wipe(struct scm_creds *creds)
> +{
> +	creds->has_creds = false;
>  }
>  
>  static __inline__ void scm_destroy(struct scm_cookie *scm)
>  {
> -	scm_destroy_cred(scm);
>  	if (scm->fp)
>  		__scm_destroy(scm);
>  }
>  
>  static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
> -			       struct scm_cookie *scm, bool forcecreds)
> +			       struct scm_cookie *scm)
>  {
> -	memset(scm, 0, sizeof(*scm));
> -	if (forcecreds)
> -		scm_set_cred(scm, task_tgid(current), current_cred());
>  	unix_get_peersec_dgram(sock, scm);
>  	if (msg->msg_controllen <= 0)
>  		return 0;
> @@ -120,18 +143,22 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
>  		return;
>  	}
>  
> -	if (test_bit(SOCK_PASSCRED, &sock->flags)) {
> +	if (test_bit(SOCK_PASSCRED, &sock->flags) && scm->creds.has_creds) {
>  		struct user_namespace *current_ns = current_user_ns();
>  		struct ucred ucreds = {
> -			.pid = scm->creds.pid,
>  			.uid = from_kuid_munged(current_ns, scm->creds.uid),
>  			.gid = from_kgid_munged(current_ns, scm->creds.gid),
>  		};
> +
> +		rcu_read_lock();
> +		/* On error, this will result in pid 0. */
> +		ucreds.pid = pid_vnr(find_pid_ns(scm->creds.init_ns_pid,
> +						 &init_pid_ns));
> +		rcu_read_unlock();
> +
>  		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(ucreds), &ucreds);
>  	}
>  
> -	scm_destroy_cred(scm);
> -
>  	scm_passec(sock, msg, scm);
>  
>  	if (!scm->fp)
> diff --git a/net/core/scm.c b/net/core/scm.c
> index 905dcc6..7dd7534 100644
> --- a/net/core/scm.c
> +++ b/net/core/scm.c
> @@ -157,8 +157,7 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
>  		case SCM_CREDENTIALS:
>  		{
>  			struct ucred creds;
> -			kuid_t uid;
> -			kgid_t gid;
> +			struct pid *pid;
>  			if (cmsg->cmsg_len != CMSG_LEN(sizeof(struct ucred)))
>  				goto error;
>  			memcpy(&creds, CMSG_DATA(cmsg), sizeof(struct ucred));
> @@ -166,41 +165,25 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
>  			if (err)
>  				goto error;
>  
> -			p->creds.pid = creds.pid;
> -			if (!p->pid || pid_vnr(p->pid) != creds.pid) {
> -				struct pid *pid;
> -				err = -ESRCH;
> -				pid = find_get_pid(creds.pid);
> -				if (!pid)
> -					goto error;
> -				put_pid(p->pid);
> -				p->pid = pid;
> -			}
> -
>  			err = -EINVAL;
> -			uid = make_kuid(current_user_ns(), creds.uid);
> -			gid = make_kgid(current_user_ns(), creds.gid);
> -			if (!uid_valid(uid) || !gid_valid(gid))
> +			p->creds.uid = make_kuid(current_user_ns(), creds.uid);
> +			p->creds.gid = make_kgid(current_user_ns(), creds.gid);
> +			if (!uid_valid(p->creds.uid) ||
> +			    !gid_valid(p->creds.gid))
>  				goto error;
>  
> -			p->creds.uid = uid;
> -			p->creds.gid = gid;
> -
> -			if (!p->cred ||
> -			    !uid_eq(p->cred->euid, uid) ||
> -			    !gid_eq(p->cred->egid, gid)) {
> -				struct cred *cred;
> -				err = -ENOMEM;
> -				cred = prepare_creds();
> -				if (!cred)
> -					goto error;
> -
> -				cred->uid = cred->euid = uid;
> -				cred->gid = cred->egid = gid;
> -				if (p->cred)
> -					put_cred(p->cred);
> -				p->cred = cred;
> +			rcu_read_lock();
> +			pid = find_vpid(creds.pid);
> +			if (!pid) {
> +				rcu_read_unlock();
> +				err = -ESRCH;
> +				goto error;
>  			}
> +			p->creds.init_ns_pid = pid_nr(pid);
> +			rcu_read_unlock();
> +
> +			p->creds.has_creds = true;
> +
>  			break;
>  		}
>  		default:
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index 1e3fd5b..8245f61 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -936,7 +936,6 @@ static int netlink_unicast_kernel(struct sock *sk, struct sk_buff *skb,
>  	if (nlk->netlink_rcv != NULL) {
>  		ret = skb->len;
>  		skb_set_owner_r(skb, sk);
> -		NETLINK_CB(skb).ssk = ssk;
>  		nlk->netlink_rcv(skb);
>  		consume_skb(skb);
>  	} else {
> @@ -1368,7 +1367,7 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	u32 dst_group;
>  	struct sk_buff *skb;
>  	int err;
> -	struct scm_cookie scm;
> +	struct scm_cookie scm = SCM_COOKIE_INIT;
>  
>  	if (msg->msg_flags&MSG_OOB)
>  		return -EOPNOTSUPP;
> @@ -1376,7 +1375,8 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	if (NULL == siocb->scm)
>  		siocb->scm = &scm;
>  
> -	err = scm_send(sock, msg, siocb->scm, true);
> +	scm_creds_from_current(&siocb->scm->creds);
> +	err = scm_send(sock, msg, siocb->scm);
>  	if (err < 0)
>  		return err;
>  
> @@ -1411,7 +1411,9 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  
>  	NETLINK_CB(skb).portid	= nlk->portid;
>  	NETLINK_CB(skb).dst_group = dst_group;
> -	NETLINK_CB(skb).creds	= siocb->scm->creds;
> +
> +	/* This is mandatory. See netlink_recvmsg. */
> +	NETLINK_CB(skb).creds = siocb->scm->creds;
>  
>  	err = -EFAULT;
>  	if (memcpy_fromiovec(skb_put(skb, len), msg->msg_iov, len)) {
> @@ -1504,7 +1506,11 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
>  		memset(&scm, 0, sizeof(scm));
>  		siocb->scm = &scm;
>  	}
> +	/* skbs without creds are from the kernel. */
>  	siocb->scm->creds = *NETLINK_CREDS(skb);
> +	if (!siocb->scm->creds.has_creds)
> +		scm_creds_from_kernel(&siocb->scm->creds);
> +
>  	if (flags & MSG_TRUNC)
>  		copied = data_skb->len;
>  
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 51be64f..0881739 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1338,10 +1338,8 @@ static void unix_detach_fds(struct scm_cookie *scm, struct sk_buff *skb)
>  
>  static void unix_destruct_scm(struct sk_buff *skb)
>  {
> -	struct scm_cookie scm;
> -	memset(&scm, 0, sizeof(scm));
> -	scm.pid  = UNIXCB(skb).pid;
> -	scm.cred = UNIXCB(skb).cred;
> +	struct scm_cookie scm = SCM_COOKIE_INIT;
> +	scm.creds = UNIXCB(skb).creds;
>  	if (UNIXCB(skb).fp)
>  		unix_detach_fds(&scm, skb);
>  
> @@ -1391,9 +1389,7 @@ static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool sen
>  {
>  	int err = 0;
>  
> -	UNIXCB(skb).pid  = get_pid(scm->pid);
> -	if (scm->cred)
> -		UNIXCB(skb).cred = get_cred(scm->cred);
> +	UNIXCB(skb).creds = scm->creds;
>  	UNIXCB(skb).fp = NULL;
>  	if (scm->fp && send_fds)
>  		err = unix_attach_fds(scm, skb);
> @@ -1410,13 +1406,12 @@ static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool sen
>  static void maybe_add_creds(struct sk_buff *skb, const struct socket *sock,
>  			    const struct sock *other)
>  {
> -	if (UNIXCB(skb).cred)
> +	if (UNIXCB(skb).creds.has_creds)
>  		return;
>  	if (test_bit(SOCK_PASSCRED, &sock->flags) ||
>  	    !other->sk_socket ||
>  	    test_bit(SOCK_PASSCRED, &other->sk_socket->flags)) {
> -		UNIXCB(skb).pid  = get_pid(task_tgid(current));
> -		UNIXCB(skb).cred = get_current_cred();
> +		scm_creds_from_current(&UNIXCB(skb).creds);
>  	}
>  }
>  
> @@ -1438,14 +1433,14 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	unsigned int hash;
>  	struct sk_buff *skb;
>  	long timeo;
> -	struct scm_cookie tmp_scm;
> +	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
>  	int max_level;
>  	int data_len = 0;
>  
>  	if (NULL == siocb->scm)
>  		siocb->scm = &tmp_scm;
>  	wait_for_unix_gc();
> -	err = scm_send(sock, msg, siocb->scm, false);
> +	err = scm_send(sock, msg, siocb->scm);
>  	if (err < 0)
>  		return err;
>  
> @@ -1607,14 +1602,14 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	int err, size;
>  	struct sk_buff *skb;
>  	int sent = 0;
> -	struct scm_cookie tmp_scm;
> +	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
>  	bool fds_sent = false;
>  	int max_level;
>  
>  	if (NULL == siocb->scm)
>  		siocb->scm = &tmp_scm;
>  	wait_for_unix_gc();
> -	err = scm_send(sock, msg, siocb->scm, false);
> +	err = scm_send(sock, msg, siocb->scm);
>  	if (err < 0)
>  		return err;
>  
> @@ -1765,7 +1760,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
>  			      int flags)
>  {
>  	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
> -	struct scm_cookie tmp_scm;
> +	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
>  	struct sock *sk = sock->sk;
>  	struct unix_sock *u = unix_sk(sk);
>  	int noblock = flags & MSG_DONTWAIT;
> @@ -1820,7 +1815,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
>  		siocb->scm = &tmp_scm;
>  		memset(&tmp_scm, 0, sizeof(tmp_scm));
>  	}
> -	scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
> +	siocb->scm->creds = UNIXCB(skb).creds;
>  	unix_set_secdata(siocb->scm, skb);
>  
>  	if (!(flags & MSG_PEEK)) {
> @@ -1898,7 +1893,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
>  			       int flags)
>  {
>  	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
> -	struct scm_cookie tmp_scm;
> +	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
>  	struct sock *sk = sock->sk;
>  	struct unix_sock *u = unix_sk(sk);
>  	struct sockaddr_un *sunaddr = msg->msg_name;
> @@ -1991,12 +1986,12 @@ again:
>  
>  		if (check_creds) {
>  			/* Never glue messages from different writers */
> -			if ((UNIXCB(skb).pid  != siocb->scm->pid) ||
> -			    (UNIXCB(skb).cred != siocb->scm->cred))
> +			if (!scm_creds_equal(&UNIXCB(skb).creds,
> +			                     &siocb->scm->creds));
>  				break;
>  		} else {
>  			/* Copy credentials */
> -			scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
> +			siocb->scm->creds = UNIXCB(skb).creds;
>  			check_creds = 1;
>  		}
>  
> -- 
> 1.8.1.4
> 
> _______________________________________________
> Containers mailing list
> Containers@lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/containers
> 

-- 
James Morris
<jmorris@namei.org>

Re: [PATCH 1/3] net: Clean up SCM_CREDENTIALS code

[James Morris]

(adding David Howells)


On Wed, 20 Mar 2013, Andy Lutomirski wrote:

> I was curious whether the uids, gids, and pids passed around worked
> correctly in the presence of multiple namespaces.  I gave up trying
> to figure it out: there are two copies of the pid (one of which has
> type u32, which is odd), a struct cred * (!), and a separate kuid
> and kgid.  IOW, all of the relevant data is stored twice, and it's
> unclear which copy is used when.
> 
> I also wondered what prevented a SO_CREDENTIALS message from being
> recieved when the credentials weren't filled out.  Answer: not very
> much (and there have been serious security bugs here in the past).
> 
> So just rewrite the thing to store a pid_t relative to the init pid
> ns, a kuid, and a kgid, and to explicitly track whether the data is
> filled out.
> 
> I haven't played with the secid code.  I have no idea whether it has
> similar problems.
> 
> I haven't benchmarked this, but it should be a respectable speedup
> in the cases where the credentials are in use.
> 
> Signed-off-by: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
> ---
> 
> Before, the program below printed this:
> 
> $ passcred
> My pid = 19873
> No SO_PASSCRED: uid=65534 gid=65534 pid=0  [this is a bug]
> SO_PASSCRED: uid=1000 gid=1000 pid=19873
> SO_PASSCRED, forked to pid 19874: uid=1000 gid=1000 pid=19874
> 
> # passcred
> My pid = 19886
> No SO_PASSCRED: uid=65534 gid=65534 pid=0
> SO_PASSCRED: uid=0 gid=0 pid=19886
> SO_PASSCRED, forked to pid 19887: uid=0 gid=0 pid=19887
> 
> 
> After:
> 
> # passcred
> My pid = 83
> No creds received
> SO_PASSCRED: uid=0 gid=0 pid=83
> SO_PASSCRED, forked to pid 84: uid=0 gid=0 pid=0
> 
> 
> #define _GNU_SOURCE
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <sys/socket.h>
> #include <stdlib.h>
> #include <stdint.h>
> #include <stdbool.h>
> #include <stdio.h>
> #include <string.h>
> #include <unistd.h>
> #include <err.h>
> 
> static void send_str(int fd, const char *msg)
> {
> 	if (send(fd, msg, strlen(msg)+1, 0) < 0)
> 		err(1, "send");
> }
> 
> int main()
> {
> 	int fds[2];
> 	if (socketpair(AF_UNIX, SOCK_DGRAM, 0, fds))
> 		err(1, "socketpair");
> 
> 	send_str(fds[1], "No SO_PASSCRED");
> 
> 	int one = 1;
> 	if (setsockopt(fds[0], SOL_SOCKET, SO_PASSCRED, &one, sizeof(one)) != 0)
> 		err(1, "SO_PASSCRED");
> 
> 	send_str(fds[1], "SO_PASSCRED");
> 
> 	if (fork() == 0) {
> 		char msg[1024];
> 		sprintf(msg, "SO_PASSCRED, forked to pid %ld", (long)getpid());
> 		send_str(fds[1], msg);
> 		return 0;
> 	}
> 
> 	int status;
> 	wait(&status);
> 	printf("My pid = %ld\n", getpid());
> 
> 	for (int i = 0; i < 3; i++) {
> 		char buf[1024];
> 		char cbuf[CMSG_SPACE(sizeof(struct ucred))];
> 		struct iovec iov;
> 		iov.iov_base = &buf;
> 		iov.iov_len = sizeof(buf);
> 		struct msghdr hdr;
> 		memset(&hdr, 0, sizeof(hdr));
> 		hdr.msg_iov = &iov;
> 		hdr.msg_iovlen = 1;
> 		hdr.msg_control = cbuf;
> 		hdr.msg_controllen = sizeof(cbuf);
> 		ssize_t bytes = recvmsg(fds[0], &hdr, 0);
> 		if (bytes < 0)
> 			err(1, "recvmsg");
> 
> 		if (hdr.msg_flags & (MSG_TRUNC | MSG_CTRUNC))
> 			errx(1, "truncated");
> 
> 		struct ucred cred;
> 		bool ok = false;
> 
> 		for (struct cmsghdr *cmsg = CMSG_FIRSTHDR(&hdr); cmsg; cmsg = CMSG_NXTHDR(&hdr, cmsg)) {
> 			if (cmsg->cmsg_level == SOL_SOCKET &&
> 			    cmsg->cmsg_type == SCM_CREDENTIALS) {
> 				cred = *((struct ucred *)CMSG_DATA(cmsg));
> 				ok = true;
> 				break;
> 			}
> 		}
> 		if (!ok) {
> 			printf("No creds received\n");
> 		} else {
> 			printf("%s: uid=%ld gid=%ld pid=%ld\n",
> 			       buf, (long)cred.uid, (long)cred.gid, (long)cred.pid);
> 		}
> 	}
> 
> 	return 0;
> }
> 
> 
>  include/net/af_unix.h    |  6 ++--
>  include/net/scm.h        | 83 ++++++++++++++++++++++++++++++++----------------
>  net/core/scm.c           | 49 ++++++++++------------------
>  net/netlink/af_netlink.c | 14 +++++---
>  net/unix/af_unix.c       | 35 +++++++++-----------
>  5 files changed, 99 insertions(+), 88 deletions(-)
> 
> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
> index 0a996a3..7874f3e 100644
> --- a/include/net/af_unix.h
> +++ b/include/net/af_unix.h
> @@ -27,12 +27,12 @@ struct unix_address {
>  	struct sockaddr_un name[0];
>  };
>  
> +/* This structure is identical to struct scm_cookie. */
>  struct unix_skb_parms {
> -	struct pid		*pid;		/* Skb credentials	*/
> -	const struct cred	*cred;
> +	struct scm_creds	creds;
>  	struct scm_fp_list	*fp;		/* Passed files		*/
>  #ifdef CONFIG_SECURITY_NETWORK
> -	u32			secid;		/* Security ID		*/
> +	u32			secid;		/* Passed security ID 	*/
>  #endif
>  };
>  
> diff --git a/include/net/scm.h b/include/net/scm.h
> index 975cca0..f6f0626 100644
> --- a/include/net/scm.h
> +++ b/include/net/scm.h
> @@ -13,9 +13,24 @@
>  #define SCM_MAX_FD	253
>  
>  struct scm_creds {
> -	u32	pid;
> -	kuid_t	uid;
> -	kgid_t	gid;
> +	bool has_creds;
> +
> +	/*
> +	 * Keeping reference counts (as to a struct pid *) in here is
> +	 * annoying -- things like skb_set_owner_[rw] and skb_clone assume
> +	 * that it's ok to memcpy skb->cb around.
> +	 *
> +	 * Fortunately (?) anything that uses the pid field in SCM_CREDENTIALS
> +	 * is fundamentally racy, since the networking code certainly isn't
> +	 * going to keep a reference alive *after* recvmsg.  So let's embrace
> +	 * the race condition at the cost of an extra hash lookup on receive.
> +	 *
> +	 * (There's an added benefit here: this approach doesn't write to
> +	 * any shared cachelines.)
> +	 */
> +	pid_t		init_ns_pid;
> +	kuid_t		uid;
> +	kgid_t		gid;
>  };
>  
>  struct scm_fp_list {
> @@ -25,15 +40,15 @@ struct scm_fp_list {
>  };
>  
>  struct scm_cookie {
> -	struct pid		*pid;		/* Skb credentials */
> -	const struct cred	*cred;
> +	struct scm_creds	creds;
>  	struct scm_fp_list	*fp;		/* Passed files		*/
> -	struct scm_creds	creds;		/* Skb credentials	*/
>  #ifdef CONFIG_SECURITY_NETWORK
>  	u32			secid;		/* Passed security ID 	*/
>  #endif
>  };
>  
> +#define SCM_COOKIE_INIT {}  /* All zeros is good. */
> +
>  extern void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm);
>  extern void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm);
>  extern int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *scm);
> @@ -50,39 +65,47 @@ static __inline__ void unix_get_peersec_dgram(struct socket *sock, struct scm_co
>  { }
>  #endif /* CONFIG_SECURITY_NETWORK */
>  
> -static __inline__ void scm_set_cred(struct scm_cookie *scm,
> -				    struct pid *pid, const struct cred *cred)
> +static __inline__ bool scm_creds_equal(const struct scm_creds *a,
> +                                       const struct scm_creds *b)
>  {
> -	scm->pid  = get_pid(pid);
> -	scm->cred = cred ? get_cred(cred) : NULL;
> -	scm->creds.pid = pid_vnr(pid);
> -	scm->creds.uid = cred ? cred->euid : INVALID_UID;
> -	scm->creds.gid = cred ? cred->egid : INVALID_GID;
> +	if (a->has_creds)
> +		return a->init_ns_pid == b->init_ns_pid &&
> +			uid_eq(a->uid, b->uid) && gid_eq(a->gid, b->gid);
> +	else
> +		return !b->has_creds;
>  }
>  
> -static __inline__ void scm_destroy_cred(struct scm_cookie *scm)
> +static __inline__ void scm_creds_from_current(struct scm_creds *creds)
>  {
> -	put_pid(scm->pid);
> -	scm->pid  = NULL;
> +	const struct cred *cred = get_current_cred();
> +	creds->has_creds = true;
> +	creds->init_ns_pid = task_tgid_nr(current);
> +	creds->uid = cred->uid;
> +	creds->gid = cred->gid;
> +}
>  
> -	if (scm->cred)
> -		put_cred(scm->cred);
> -	scm->cred = NULL;
> +static __inline__ void scm_creds_from_kernel(struct scm_creds *creds)
> +{
> +	creds->has_creds = true;
> +	creds->init_ns_pid = 0;
> +	creds->uid = GLOBAL_ROOT_UID;
> +	creds->gid = GLOBAL_ROOT_GID;
> +}
> +
> +static __inline__ void scm_creds_wipe(struct scm_creds *creds)
> +{
> +	creds->has_creds = false;
>  }
>  
>  static __inline__ void scm_destroy(struct scm_cookie *scm)
>  {
> -	scm_destroy_cred(scm);
>  	if (scm->fp)
>  		__scm_destroy(scm);
>  }
>  
>  static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
> -			       struct scm_cookie *scm, bool forcecreds)
> +			       struct scm_cookie *scm)
>  {
> -	memset(scm, 0, sizeof(*scm));
> -	if (forcecreds)
> -		scm_set_cred(scm, task_tgid(current), current_cred());
>  	unix_get_peersec_dgram(sock, scm);
>  	if (msg->msg_controllen <= 0)
>  		return 0;
> @@ -120,18 +143,22 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
>  		return;
>  	}
>  
> -	if (test_bit(SOCK_PASSCRED, &sock->flags)) {
> +	if (test_bit(SOCK_PASSCRED, &sock->flags) && scm->creds.has_creds) {
>  		struct user_namespace *current_ns = current_user_ns();
>  		struct ucred ucreds = {
> -			.pid = scm->creds.pid,
>  			.uid = from_kuid_munged(current_ns, scm->creds.uid),
>  			.gid = from_kgid_munged(current_ns, scm->creds.gid),
>  		};
> +
> +		rcu_read_lock();
> +		/* On error, this will result in pid 0. */
> +		ucreds.pid = pid_vnr(find_pid_ns(scm->creds.init_ns_pid,
> +						 &init_pid_ns));
> +		rcu_read_unlock();
> +
>  		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(ucreds), &ucreds);
>  	}
>  
> -	scm_destroy_cred(scm);
> -
>  	scm_passec(sock, msg, scm);
>  
>  	if (!scm->fp)
> diff --git a/net/core/scm.c b/net/core/scm.c
> index 905dcc6..7dd7534 100644
> --- a/net/core/scm.c
> +++ b/net/core/scm.c
> @@ -157,8 +157,7 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
>  		case SCM_CREDENTIALS:
>  		{
>  			struct ucred creds;
> -			kuid_t uid;
> -			kgid_t gid;
> +			struct pid *pid;
>  			if (cmsg->cmsg_len != CMSG_LEN(sizeof(struct ucred)))
>  				goto error;
>  			memcpy(&creds, CMSG_DATA(cmsg), sizeof(struct ucred));
> @@ -166,41 +165,25 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
>  			if (err)
>  				goto error;
>  
> -			p->creds.pid = creds.pid;
> -			if (!p->pid || pid_vnr(p->pid) != creds.pid) {
> -				struct pid *pid;
> -				err = -ESRCH;
> -				pid = find_get_pid(creds.pid);
> -				if (!pid)
> -					goto error;
> -				put_pid(p->pid);
> -				p->pid = pid;
> -			}
> -
>  			err = -EINVAL;
> -			uid = make_kuid(current_user_ns(), creds.uid);
> -			gid = make_kgid(current_user_ns(), creds.gid);
> -			if (!uid_valid(uid) || !gid_valid(gid))
> +			p->creds.uid = make_kuid(current_user_ns(), creds.uid);
> +			p->creds.gid = make_kgid(current_user_ns(), creds.gid);
> +			if (!uid_valid(p->creds.uid) ||
> +			    !gid_valid(p->creds.gid))
>  				goto error;
>  
> -			p->creds.uid = uid;
> -			p->creds.gid = gid;
> -
> -			if (!p->cred ||
> -			    !uid_eq(p->cred->euid, uid) ||
> -			    !gid_eq(p->cred->egid, gid)) {
> -				struct cred *cred;
> -				err = -ENOMEM;
> -				cred = prepare_creds();
> -				if (!cred)
> -					goto error;
> -
> -				cred->uid = cred->euid = uid;
> -				cred->gid = cred->egid = gid;
> -				if (p->cred)
> -					put_cred(p->cred);
> -				p->cred = cred;
> +			rcu_read_lock();
> +			pid = find_vpid(creds.pid);
> +			if (!pid) {
> +				rcu_read_unlock();
> +				err = -ESRCH;
> +				goto error;
>  			}
> +			p->creds.init_ns_pid = pid_nr(pid);
> +			rcu_read_unlock();
> +
> +			p->creds.has_creds = true;
> +
>  			break;
>  		}
>  		default:
> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
> index 1e3fd5b..8245f61 100644
> --- a/net/netlink/af_netlink.c
> +++ b/net/netlink/af_netlink.c
> @@ -936,7 +936,6 @@ static int netlink_unicast_kernel(struct sock *sk, struct sk_buff *skb,
>  	if (nlk->netlink_rcv != NULL) {
>  		ret = skb->len;
>  		skb_set_owner_r(skb, sk);
> -		NETLINK_CB(skb).ssk = ssk;
>  		nlk->netlink_rcv(skb);
>  		consume_skb(skb);
>  	} else {
> @@ -1368,7 +1367,7 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	u32 dst_group;
>  	struct sk_buff *skb;
>  	int err;
> -	struct scm_cookie scm;
> +	struct scm_cookie scm = SCM_COOKIE_INIT;
>  
>  	if (msg->msg_flags&MSG_OOB)
>  		return -EOPNOTSUPP;
> @@ -1376,7 +1375,8 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	if (NULL == siocb->scm)
>  		siocb->scm = &scm;
>  
> -	err = scm_send(sock, msg, siocb->scm, true);
> +	scm_creds_from_current(&siocb->scm->creds);
> +	err = scm_send(sock, msg, siocb->scm);
>  	if (err < 0)
>  		return err;
>  
> @@ -1411,7 +1411,9 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  
>  	NETLINK_CB(skb).portid	= nlk->portid;
>  	NETLINK_CB(skb).dst_group = dst_group;
> -	NETLINK_CB(skb).creds	= siocb->scm->creds;
> +
> +	/* This is mandatory. See netlink_recvmsg. */
> +	NETLINK_CB(skb).creds = siocb->scm->creds;
>  
>  	err = -EFAULT;
>  	if (memcpy_fromiovec(skb_put(skb, len), msg->msg_iov, len)) {
> @@ -1504,7 +1506,11 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
>  		memset(&scm, 0, sizeof(scm));
>  		siocb->scm = &scm;
>  	}
> +	/* skbs without creds are from the kernel. */
>  	siocb->scm->creds = *NETLINK_CREDS(skb);
> +	if (!siocb->scm->creds.has_creds)
> +		scm_creds_from_kernel(&siocb->scm->creds);
> +
>  	if (flags & MSG_TRUNC)
>  		copied = data_skb->len;
>  
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 51be64f..0881739 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1338,10 +1338,8 @@ static void unix_detach_fds(struct scm_cookie *scm, struct sk_buff *skb)
>  
>  static void unix_destruct_scm(struct sk_buff *skb)
>  {
> -	struct scm_cookie scm;
> -	memset(&scm, 0, sizeof(scm));
> -	scm.pid  = UNIXCB(skb).pid;
> -	scm.cred = UNIXCB(skb).cred;
> +	struct scm_cookie scm = SCM_COOKIE_INIT;
> +	scm.creds = UNIXCB(skb).creds;
>  	if (UNIXCB(skb).fp)
>  		unix_detach_fds(&scm, skb);
>  
> @@ -1391,9 +1389,7 @@ static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool sen
>  {
>  	int err = 0;
>  
> -	UNIXCB(skb).pid  = get_pid(scm->pid);
> -	if (scm->cred)
> -		UNIXCB(skb).cred = get_cred(scm->cred);
> +	UNIXCB(skb).creds = scm->creds;
>  	UNIXCB(skb).fp = NULL;
>  	if (scm->fp && send_fds)
>  		err = unix_attach_fds(scm, skb);
> @@ -1410,13 +1406,12 @@ static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool sen
>  static void maybe_add_creds(struct sk_buff *skb, const struct socket *sock,
>  			    const struct sock *other)
>  {
> -	if (UNIXCB(skb).cred)
> +	if (UNIXCB(skb).creds.has_creds)
>  		return;
>  	if (test_bit(SOCK_PASSCRED, &sock->flags) ||
>  	    !other->sk_socket ||
>  	    test_bit(SOCK_PASSCRED, &other->sk_socket->flags)) {
> -		UNIXCB(skb).pid  = get_pid(task_tgid(current));
> -		UNIXCB(skb).cred = get_current_cred();
> +		scm_creds_from_current(&UNIXCB(skb).creds);
>  	}
>  }
>  
> @@ -1438,14 +1433,14 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	unsigned int hash;
>  	struct sk_buff *skb;
>  	long timeo;
> -	struct scm_cookie tmp_scm;
> +	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
>  	int max_level;
>  	int data_len = 0;
>  
>  	if (NULL == siocb->scm)
>  		siocb->scm = &tmp_scm;
>  	wait_for_unix_gc();
> -	err = scm_send(sock, msg, siocb->scm, false);
> +	err = scm_send(sock, msg, siocb->scm);
>  	if (err < 0)
>  		return err;
>  
> @@ -1607,14 +1602,14 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
>  	int err, size;
>  	struct sk_buff *skb;
>  	int sent = 0;
> -	struct scm_cookie tmp_scm;
> +	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
>  	bool fds_sent = false;
>  	int max_level;
>  
>  	if (NULL == siocb->scm)
>  		siocb->scm = &tmp_scm;
>  	wait_for_unix_gc();
> -	err = scm_send(sock, msg, siocb->scm, false);
> +	err = scm_send(sock, msg, siocb->scm);
>  	if (err < 0)
>  		return err;
>  
> @@ -1765,7 +1760,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
>  			      int flags)
>  {
>  	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
> -	struct scm_cookie tmp_scm;
> +	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
>  	struct sock *sk = sock->sk;
>  	struct unix_sock *u = unix_sk(sk);
>  	int noblock = flags & MSG_DONTWAIT;
> @@ -1820,7 +1815,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
>  		siocb->scm = &tmp_scm;
>  		memset(&tmp_scm, 0, sizeof(tmp_scm));
>  	}
> -	scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
> +	siocb->scm->creds = UNIXCB(skb).creds;
>  	unix_set_secdata(siocb->scm, skb);
>  
>  	if (!(flags & MSG_PEEK)) {
> @@ -1898,7 +1893,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
>  			       int flags)
>  {
>  	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
> -	struct scm_cookie tmp_scm;
> +	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
>  	struct sock *sk = sock->sk;
>  	struct unix_sock *u = unix_sk(sk);
>  	struct sockaddr_un *sunaddr = msg->msg_name;
> @@ -1991,12 +1986,12 @@ again:
>  
>  		if (check_creds) {
>  			/* Never glue messages from different writers */
> -			if ((UNIXCB(skb).pid  != siocb->scm->pid) ||
> -			    (UNIXCB(skb).cred != siocb->scm->cred))
> +			if (!scm_creds_equal(&UNIXCB(skb).creds,
> +			                     &siocb->scm->creds));
>  				break;
>  		} else {
>  			/* Copy credentials */
> -			scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
> +			siocb->scm->creds = UNIXCB(skb).creds;
>  			check_creds = 1;
>  		}
>  
> -- 
> 1.8.1.4
> 
> _______________________________________________
> Containers mailing list
> Containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org
> https://lists.linuxfoundation.org/mailman/listinfo/containers
> 

-- 
James Morris
<jmorris-gx6/JNMH7DfYtjvyW6yDsg@public.gmane.org>

Re: [PATCH 1/3] net: Clean up SCM_CREDENTIALS code

[Eric W. Biederman]

Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> writes:

> I was curious whether the uids, gids, and pids passed around worked
> correctly in the presence of multiple namespaces.  I gave up trying
> to figure it out: there are two copies of the pid (one of which has
> type u32, which is odd), a struct cred * (!), and a separate kuid
> and kgid.  IOW, all of the relevant data is stored twice, and it's
> unclear which copy is used when.
>
> I also wondered what prevented a SO_CREDENTIALS message from being
> recieved when the credentials weren't filled out.  Answer: not very
> much (and there have been serious security bugs here in the past).
>
> So just rewrite the thing to store a pid_t relative to the init pid
> ns, a kuid, and a kgid, and to explicitly track whether the data is
> filled out.
>
> I haven't played with the secid code.  I have no idea whether it has
> similar problems.
>
> I haven't benchmarked this, but it should be a respectable speedup
> in the cases where the credentials are in use.

The basic principle of no longer passing the struct cred we can
certainly do.

I am less convinced about the struct pid, but arguably that is the
proper approach.

A patch that proclaims that you didn't understand what the code was
doing but you changed it anyway, suggests there are subtle bugs
in there that you overlooked.

Certainly killing NETLINK_CB(sbk).ssk is a bug.

I do think there is a lot of good stuff in here and if you break this up
into smaller patches simpler patches, and keep an eye on the speed of
sending things messages without credentials.  I am pretty certain you
can cook up something that is mergable.

Eric

Re: [PATCH 1/3] net: Clean up SCM_CREDENTIALS code

[Eric W. Biederman]

Andy Lutomirski <luto@amacapital.net> writes:

> I was curious whether the uids, gids, and pids passed around worked
> correctly in the presence of multiple namespaces.  I gave up trying
> to figure it out: there are two copies of the pid (one of which has
> type u32, which is odd), a struct cred * (!), and a separate kuid
> and kgid.  IOW, all of the relevant data is stored twice, and it's
> unclear which copy is used when.
>
> I also wondered what prevented a SO_CREDENTIALS message from being
> recieved when the credentials weren't filled out.  Answer: not very
> much (and there have been serious security bugs here in the past).
>
> So just rewrite the thing to store a pid_t relative to the init pid
> ns, a kuid, and a kgid, and to explicitly track whether the data is
> filled out.
>
> I haven't played with the secid code.  I have no idea whether it has
> similar problems.
>
> I haven't benchmarked this, but it should be a respectable speedup
> in the cases where the credentials are in use.

The basic principle of no longer passing the struct cred we can
certainly do.

I am less convinced about the struct pid, but arguably that is the
proper approach.

A patch that proclaims that you didn't understand what the code was
doing but you changed it anyway, suggests there are subtle bugs
in there that you overlooked.

Certainly killing NETLINK_CB(sbk).ssk is a bug.

I do think there is a lot of good stuff in here and if you break this up
into smaller patches simpler patches, and keep an eye on the speed of
sending things messages without credentials.  I am pretty certain you
can cook up something that is mergable.

Eric

Re: [PATCH 1/3] net: Clean up SCM_CREDENTIALS code

[Andy Lutomirski]

On Wed, Mar 20, 2013 at 11:54 PM, Eric W. Biederman
<ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org> wrote:
> Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> writes:
>
>> I was curious whether the uids, gids, and pids passed around worked
>> correctly in the presence of multiple namespaces.  I gave up trying
>> to figure it out: there are two copies of the pid (one of which has
>> type u32, which is odd), a struct cred * (!), and a separate kuid
>> and kgid.  IOW, all of the relevant data is stored twice, and it's
>> unclear which copy is used when.
>>
>> I also wondered what prevented a SO_CREDENTIALS message from being
>> recieved when the credentials weren't filled out.  Answer: not very
>> much (and there have been serious security bugs here in the past).
>>
>> So just rewrite the thing to store a pid_t relative to the init pid
>> ns, a kuid, and a kgid, and to explicitly track whether the data is
>> filled out.
>>
>> I haven't played with the secid code.  I have no idea whether it has
>> similar problems.
>>
>> I haven't benchmarked this, but it should be a respectable speedup
>> in the cases where the credentials are in use.
>
> The basic principle of no longer passing the struct cred we can
> certainly do.
>
> I am less convinced about the struct pid, but arguably that is the
> proper approach.

I agree it's not pretty.  OTOH it's faster, simpler, and I don't see
any benefit of keeping an explicit struct pid reference.  With this
approach, the only way to attack the code is to get a pid to be reused
or to impersonate pid 0 and try to confuse something.  But the other
way has the same issue, just with a shorter race window.

>
> A patch that proclaims that you didn't understand what the code was
> doing but you changed it anyway, suggests there are subtle bugs
> in there that you overlooked.

I'll improve the changelog text.  After following the code around, I
now understand what was going on.

>
> Certainly killing NETLINK_CB(sbk).ssk is a bug.

As noted in my other email, I'll drop that patch entirely.

>
> I do think there is a lot of good stuff in here and if you break this up
> into smaller patches simpler patches, and keep an eye on the speed of
> sending things messages without credentials.  I am pretty certain you
> can cook up something that is mergable.

I'm not sure how to split it up more without making it messier.  Once
the data structure changes, most of the rest of the changes have to
come along.

In any case, I won't send out a new version until I get some comments
on the code (other than the ssk thing).

--Andy

[PATCH 2/3] netlink: Remove an unused pointer in netlink_skb_parms

[Andy Lutomirski]

Signed-off-by: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
---
 include/linux/netlink.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index e0f746b..9ac1201 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -19,7 +19,6 @@ struct netlink_skb_parms {
 	struct scm_creds	creds;		/* Skb credentials	*/
 	__u32			portid;
 	__u32			dst_group;
-	struct sock		*ssk;
 };
 
 #define NETLINK_CB(skb)		(*(struct netlink_skb_parms*)&((skb)->cb))
-- 
1.8.1.4

Re: [PATCH 2/3] netlink: Remove an unused pointer in netlink_skb_parms

[Eric W. Biederman]

net/ipv4/inet_diag.c:			   sk_user_ns(NETLINK_CB(in_skb).ssk),
net/ipv4/inet_diag.c:				  sk_user_ns(NETLINK_CB(cb->skb).ssk),
net/ipv4/inet_diag.c:					       sk_user_ns(NETLINK_CB(cb->skb).ssk),
net/ipv4/udp_diag.c:			sk_user_ns(NETLINK_CB(cb->skb).ssk),
net/ipv4/udp_diag.c:			   sk_user_ns(NETLINK_CB(in_skb).ssk),
net/netfilter/nfnetlink_log.c:					       sk_user_ns(NETLINK_CB(skb).ssk));
net/netlink/af_netlink.c:		NETLINK_CB(skb).ssk = ssk;
net/sched/cls_flow.c:		    sk_user_ns(NETLINK_CB(in_skb).ssk) != &init_user_ns)

I count 8 uses.

Eric

>
> diff --git a/include/linux/netlink.h b/include/linux/netlink.h
> index e0f746b..9ac1201 100644
> --- a/include/linux/netlink.h
> +++ b/include/linux/netlink.h
> @@ -19,7 +19,6 @@ struct netlink_skb_parms {
>  	struct scm_creds	creds;		/* Skb credentials	*/
>  	__u32			portid;
>  	__u32			dst_group;
> -	struct sock		*ssk;
>  };
>  
>  #define NETLINK_CB(skb)		(*(struct netlink_skb_parms*)&((skb)->cb))

Re: [PATCH 2/3] netlink: Remove an unused pointer in netlink_skb_parms

[Andy Lutomirski]

On Wed, Mar 20, 2013 at 11:36 PM, Eric W. Biederman
<ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org> wrote:
>
> net/ipv4/inet_diag.c:                      sk_user_ns(NETLINK_CB(in_skb).ssk),
> net/ipv4/inet_diag.c:                             sk_user_ns(NETLINK_CB(cb->skb).ssk),
> net/ipv4/inet_diag.c:                                          sk_user_ns(NETLINK_CB(cb->skb).ssk),
> net/ipv4/udp_diag.c:                    sk_user_ns(NETLINK_CB(cb->skb).ssk),
> net/ipv4/udp_diag.c:                       sk_user_ns(NETLINK_CB(in_skb).ssk),
> net/netfilter/nfnetlink_log.c:                                         sk_user_ns(NETLINK_CB(skb).ssk));
> net/netlink/af_netlink.c:               NETLINK_CB(skb).ssk = ssk;
> net/sched/cls_flow.c:               sk_user_ns(NETLINK_CB(in_skb).ssk) != &init_user_ns)
>
> I count 8 uses.

Whoops.  I clearly fail at grepping and building with the correct
configuration.  I'll drop this patch entirely -- it's independent of
the other two.

>
> Eric
>
>>
>> diff --git a/include/linux/netlink.h b/include/linux/netlink.h
>> index e0f746b..9ac1201 100644
>> --- a/include/linux/netlink.h
>> +++ b/include/linux/netlink.h
>> @@ -19,7 +19,6 @@ struct netlink_skb_parms {
>>       struct scm_creds        creds;          /* Skb credentials      */
>>       __u32                   portid;
>>       __u32                   dst_group;
>> -     struct sock             *ssk;
>>  };
>>
>>  #define NETLINK_CB(skb)              (*(struct netlink_skb_parms*)&((skb)->cb))



-- 
Andy Lutomirski
AMA Capital Management, LLC

[PATCH 3/3] net: Remove sock_iocb.scm

[Andy Lutomirski]

I'm not sure why this was here.  Any use after sendmsg or recvmsg
returned was bogus anyway.

Signed-off-by: Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
---
 include/net/sock.h       |  1 -
 net/netlink/af_netlink.c | 27 +++++++-------------
 net/socket.c             |  2 --
 net/unix/af_unix.c       | 66 ++++++++++++++++++------------------------------
 4 files changed, 33 insertions(+), 63 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 14f6e9d..87b134e 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1299,7 +1299,6 @@ struct sock_iocb {
 	int			size;
 	struct socket		*sock;
 	struct sock		*sk;
-	struct scm_cookie	*scm;
 	struct msghdr		*msg, async_msg;
 	struct kiocb		*kiocb;
 };
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 8245f61..874bc1f 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1359,7 +1359,6 @@ static void netlink_cmsg_recv_pktinfo(struct msghdr *msg, struct sk_buff *skb)
 static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 			   struct msghdr *msg, size_t len)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(kiocb);
 	struct sock *sk = sock->sk;
 	struct netlink_sock *nlk = nlk_sk(sk);
 	struct sockaddr_nl *addr = msg->msg_name;
@@ -1372,11 +1371,8 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	if (msg->msg_flags&MSG_OOB)
 		return -EOPNOTSUPP;
 
-	if (NULL == siocb->scm)
-		siocb->scm = &scm;
-
-	scm_creds_from_current(&siocb->scm->creds);
-	err = scm_send(sock, msg, siocb->scm);
+	scm_creds_from_current(&scm.creds);
+	err = scm_send(sock, msg, &scm);
 	if (err < 0)
 		return err;
 
@@ -1413,7 +1409,7 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	NETLINK_CB(skb).dst_group = dst_group;
 
 	/* This is mandatory. See netlink_recvmsg. */
-	NETLINK_CB(skb).creds = siocb->scm->creds;
+	NETLINK_CB(skb).creds = scm.creds;
 
 	err = -EFAULT;
 	if (memcpy_fromiovec(skb_put(skb, len), msg->msg_iov, len)) {
@@ -1434,7 +1430,7 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	err = netlink_unicast(sk, skb, dst_portid, msg->msg_flags&MSG_DONTWAIT);
 
 out:
-	scm_destroy(siocb->scm);
+	scm_destroy(&scm);
 	return err;
 }
 
@@ -1442,8 +1438,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 			   struct msghdr *msg, size_t len,
 			   int flags)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(kiocb);
-	struct scm_cookie scm;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	struct sock *sk = sock->sk;
 	struct netlink_sock *nlk = nlk_sk(sk);
 	int noblock = flags&MSG_DONTWAIT;
@@ -1502,14 +1497,10 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 	if (nlk->flags & NETLINK_RECV_PKTINFO)
 		netlink_cmsg_recv_pktinfo(msg, skb);
 
-	if (NULL == siocb->scm) {
-		memset(&scm, 0, sizeof(scm));
-		siocb->scm = &scm;
-	}
 	/* skbs without creds are from the kernel. */
-	siocb->scm->creds = *NETLINK_CREDS(skb);
-	if (!siocb->scm->creds.has_creds)
-		scm_creds_from_kernel(&siocb->scm->creds);
+	scm.creds = *NETLINK_CREDS(skb);
+	if (!scm.creds.has_creds)
+		scm_creds_from_kernel(&scm.creds);
 
 	if (flags & MSG_TRUNC)
 		copied = data_skb->len;
@@ -1524,7 +1515,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 		}
 	}
 
-	scm_recv(sock, msg, siocb->scm, flags);
+	scm_recv(sock, msg, &scm, flags);
 out:
 	netlink_rcv_wake(sk);
 	return err ? : copied;
diff --git a/net/socket.c b/net/socket.c
index 88f759a..26a65b4 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -619,7 +619,6 @@ static inline int __sock_sendmsg_nosec(struct kiocb *iocb, struct socket *sock,
 	struct sock_iocb *si = kiocb_to_siocb(iocb);
 
 	si->sock = sock;
-	si->scm = NULL;
 	si->msg = msg;
 	si->size = size;
 
@@ -781,7 +780,6 @@ static inline int __sock_recvmsg_nosec(struct kiocb *iocb, struct socket *sock,
 	struct sock_iocb *si = kiocb_to_siocb(iocb);
 
 	si->sock = sock;
-	si->scm = NULL;
 	si->msg = msg;
 	si->size = size;
 	si->flags = flags;
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 0881739..e6541c1 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1422,7 +1422,6 @@ static void maybe_add_creds(struct sk_buff *skb, const struct socket *sock,
 static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 			      struct msghdr *msg, size_t len)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(kiocb);
 	struct sock *sk = sock->sk;
 	struct net *net = sock_net(sk);
 	struct unix_sock *u = unix_sk(sk);
@@ -1433,14 +1432,12 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	unsigned int hash;
 	struct sk_buff *skb;
 	long timeo;
-	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	int max_level;
 	int data_len = 0;
 
-	if (NULL == siocb->scm)
-		siocb->scm = &tmp_scm;
 	wait_for_unix_gc();
-	err = scm_send(sock, msg, siocb->scm);
+	err = scm_send(sock, msg, &scm);
 	if (err < 0)
 		return err;
 
@@ -1479,11 +1476,11 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	if (skb == NULL)
 		goto out;
 
-	err = unix_scm_to_skb(siocb->scm, skb, true);
+	err = unix_scm_to_skb(&scm, skb, true);
 	if (err < 0)
 		goto out_free;
 	max_level = err + 1;
-	unix_get_secdata(siocb->scm, skb);
+	unix_get_secdata(&scm, skb);
 
 	skb_put(skb, len - data_len);
 	skb->data_len = data_len;
@@ -1578,7 +1575,7 @@ restart:
 	unix_state_unlock(other);
 	other->sk_data_ready(other, len);
 	sock_put(other);
-	scm_destroy(siocb->scm);
+	scm_destroy(&scm);
 	return len;
 
 out_unlock:
@@ -1588,7 +1585,7 @@ out_free:
 out:
 	if (other)
 		sock_put(other);
-	scm_destroy(siocb->scm);
+	scm_destroy(&scm);
 	return err;
 }
 
@@ -1596,20 +1593,17 @@ out:
 static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 			       struct msghdr *msg, size_t len)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(kiocb);
 	struct sock *sk = sock->sk;
 	struct sock *other = NULL;
 	int err, size;
 	struct sk_buff *skb;
 	int sent = 0;
-	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	bool fds_sent = false;
 	int max_level;
 
-	if (NULL == siocb->scm)
-		siocb->scm = &tmp_scm;
 	wait_for_unix_gc();
-	err = scm_send(sock, msg, siocb->scm);
+	err = scm_send(sock, msg, &scm);
 	if (err < 0)
 		return err;
 
@@ -1666,7 +1660,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 
 
 		/* Only send the fds in the first buffer */
-		err = unix_scm_to_skb(siocb->scm, skb, !fds_sent);
+		err = unix_scm_to_skb(&scm, skb, !fds_sent);
 		if (err < 0) {
 			kfree_skb(skb);
 			goto out_err;
@@ -1695,8 +1689,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		sent += size;
 	}
 
-	scm_destroy(siocb->scm);
-	siocb->scm = NULL;
+	scm_destroy(&scm);
 
 	return sent;
 
@@ -1708,8 +1701,7 @@ pipe_err:
 		send_sig(SIGPIPE, current, 0);
 	err = -EPIPE;
 out_err:
-	scm_destroy(siocb->scm);
-	siocb->scm = NULL;
+	scm_destroy(&scm);
 	return sent ? : err;
 }
 
@@ -1760,7 +1752,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 			      int flags)
 {
 	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
-	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
 	int noblock = flags & MSG_DONTWAIT;
@@ -1811,16 +1803,12 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 	if (sock_flag(sk, SOCK_RCVTSTAMP))
 		__sock_recv_timestamp(msg, sk, skb);
 
-	if (!siocb->scm) {
-		siocb->scm = &tmp_scm;
-		memset(&tmp_scm, 0, sizeof(tmp_scm));
-	}
-	siocb->scm->creds = UNIXCB(skb).creds;
-	unix_set_secdata(siocb->scm, skb);
+	scm.creds = UNIXCB(skb).creds;
+	unix_set_secdata(&scm, skb);
 
 	if (!(flags & MSG_PEEK)) {
 		if (UNIXCB(skb).fp)
-			unix_detach_fds(siocb->scm, skb);
+			unix_detach_fds(&scm, skb);
 
 		sk_peek_offset_bwd(sk, skb->len);
 	} else {
@@ -1840,11 +1828,11 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 		sk_peek_offset_fwd(sk, size);
 
 		if (UNIXCB(skb).fp)
-			siocb->scm->fp = scm_fp_dup(UNIXCB(skb).fp);
+			scm.fp = scm_fp_dup(UNIXCB(skb).fp);
 	}
 	err = (flags & MSG_TRUNC) ? skb->len - skip : size;
 
-	scm_recv(sock, msg, siocb->scm, flags);
+	scm_recv(sock, msg, &scm, flags);
 
 out_free:
 	skb_free_datagram(sk, skb);
@@ -1892,8 +1880,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 			       struct msghdr *msg, size_t size,
 			       int flags)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
-	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
 	struct sockaddr_un *sunaddr = msg->msg_name;
@@ -1921,11 +1908,6 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 	 * while sleeps in memcpy_tomsg
 	 */
 
-	if (!siocb->scm) {
-		siocb->scm = &tmp_scm;
-		memset(&tmp_scm, 0, sizeof(tmp_scm));
-	}
-
 	err = mutex_lock_interruptible(&u->readlock);
 	if (err) {
 		err = sock_intr_errno(timeo);
@@ -1987,11 +1969,11 @@ again:
 		if (check_creds) {
 			/* Never glue messages from different writers */
 			if (!scm_creds_equal(&UNIXCB(skb).creds,
-			                     &siocb->scm->creds));
+			                     &scm.creds));
 				break;
 		} else {
 			/* Copy credentials */
-			siocb->scm->creds = UNIXCB(skb).creds;
+			scm.creds = UNIXCB(skb).creds;
 			check_creds = 1;
 		}
 
@@ -2017,7 +1999,7 @@ again:
 			sk_peek_offset_bwd(sk, chunk);
 
 			if (UNIXCB(skb).fp)
-				unix_detach_fds(siocb->scm, skb);
+				unix_detach_fds(&scm, skb);
 
 			if (skb->len)
 				break;
@@ -2025,13 +2007,13 @@ again:
 			skb_unlink(skb, &sk->sk_receive_queue);
 			consume_skb(skb);
 
-			if (siocb->scm->fp)
+			if (scm.fp)
 				break;
 		} else {
 			/* It is questionable, see note in unix_dgram_recvmsg.
 			 */
 			if (UNIXCB(skb).fp)
-				siocb->scm->fp = scm_fp_dup(UNIXCB(skb).fp);
+				scm.fp = scm_fp_dup(UNIXCB(skb).fp);
 
 			sk_peek_offset_fwd(sk, chunk);
 
@@ -2040,7 +2022,7 @@ again:
 	} while (size);
 
 	mutex_unlock(&u->readlock);
-	scm_recv(sock, msg, siocb->scm, flags);
+	scm_recv(sock, msg, &scm, flags);
 out:
 	return copied ? : err;
 }
-- 
1.8.1.4

[PATCH 3/3] net: Remove sock_iocb.scm

[Andy Lutomirski]

I'm not sure why this was here.  Any use after sendmsg or recvmsg
returned was bogus anyway.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 include/net/sock.h       |  1 -
 net/netlink/af_netlink.c | 27 +++++++-------------
 net/socket.c             |  2 --
 net/unix/af_unix.c       | 66 ++++++++++++++++++------------------------------
 4 files changed, 33 insertions(+), 63 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 14f6e9d..87b134e 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1299,7 +1299,6 @@ struct sock_iocb {
 	int			size;
 	struct socket		*sock;
 	struct sock		*sk;
-	struct scm_cookie	*scm;
 	struct msghdr		*msg, async_msg;
 	struct kiocb		*kiocb;
 };
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 8245f61..874bc1f 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1359,7 +1359,6 @@ static void netlink_cmsg_recv_pktinfo(struct msghdr *msg, struct sk_buff *skb)
 static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 			   struct msghdr *msg, size_t len)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(kiocb);
 	struct sock *sk = sock->sk;
 	struct netlink_sock *nlk = nlk_sk(sk);
 	struct sockaddr_nl *addr = msg->msg_name;
@@ -1372,11 +1371,8 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	if (msg->msg_flags&MSG_OOB)
 		return -EOPNOTSUPP;
 
-	if (NULL == siocb->scm)
-		siocb->scm = &scm;
-
-	scm_creds_from_current(&siocb->scm->creds);
-	err = scm_send(sock, msg, siocb->scm);
+	scm_creds_from_current(&scm.creds);
+	err = scm_send(sock, msg, &scm);
 	if (err < 0)
 		return err;
 
@@ -1413,7 +1409,7 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	NETLINK_CB(skb).dst_group = dst_group;
 
 	/* This is mandatory. See netlink_recvmsg. */
-	NETLINK_CB(skb).creds = siocb->scm->creds;
+	NETLINK_CB(skb).creds = scm.creds;
 
 	err = -EFAULT;
 	if (memcpy_fromiovec(skb_put(skb, len), msg->msg_iov, len)) {
@@ -1434,7 +1430,7 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	err = netlink_unicast(sk, skb, dst_portid, msg->msg_flags&MSG_DONTWAIT);
 
 out:
-	scm_destroy(siocb->scm);
+	scm_destroy(&scm);
 	return err;
 }
 
@@ -1442,8 +1438,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 			   struct msghdr *msg, size_t len,
 			   int flags)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(kiocb);
-	struct scm_cookie scm;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	struct sock *sk = sock->sk;
 	struct netlink_sock *nlk = nlk_sk(sk);
 	int noblock = flags&MSG_DONTWAIT;
@@ -1502,14 +1497,10 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 	if (nlk->flags & NETLINK_RECV_PKTINFO)
 		netlink_cmsg_recv_pktinfo(msg, skb);
 
-	if (NULL == siocb->scm) {
-		memset(&scm, 0, sizeof(scm));
-		siocb->scm = &scm;
-	}
 	/* skbs without creds are from the kernel. */
-	siocb->scm->creds = *NETLINK_CREDS(skb);
-	if (!siocb->scm->creds.has_creds)
-		scm_creds_from_kernel(&siocb->scm->creds);
+	scm.creds = *NETLINK_CREDS(skb);
+	if (!scm.creds.has_creds)
+		scm_creds_from_kernel(&scm.creds);
 
 	if (flags & MSG_TRUNC)
 		copied = data_skb->len;
@@ -1524,7 +1515,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 		}
 	}
 
-	scm_recv(sock, msg, siocb->scm, flags);
+	scm_recv(sock, msg, &scm, flags);
 out:
 	netlink_rcv_wake(sk);
 	return err ? : copied;
diff --git a/net/socket.c b/net/socket.c
index 88f759a..26a65b4 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -619,7 +619,6 @@ static inline int __sock_sendmsg_nosec(struct kiocb *iocb, struct socket *sock,
 	struct sock_iocb *si = kiocb_to_siocb(iocb);
 
 	si->sock = sock;
-	si->scm = NULL;
 	si->msg = msg;
 	si->size = size;
 
@@ -781,7 +780,6 @@ static inline int __sock_recvmsg_nosec(struct kiocb *iocb, struct socket *sock,
 	struct sock_iocb *si = kiocb_to_siocb(iocb);
 
 	si->sock = sock;
-	si->scm = NULL;
 	si->msg = msg;
 	si->size = size;
 	si->flags = flags;
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 0881739..e6541c1 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1422,7 +1422,6 @@ static void maybe_add_creds(struct sk_buff *skb, const struct socket *sock,
 static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 			      struct msghdr *msg, size_t len)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(kiocb);
 	struct sock *sk = sock->sk;
 	struct net *net = sock_net(sk);
 	struct unix_sock *u = unix_sk(sk);
@@ -1433,14 +1432,12 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	unsigned int hash;
 	struct sk_buff *skb;
 	long timeo;
-	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	int max_level;
 	int data_len = 0;
 
-	if (NULL == siocb->scm)
-		siocb->scm = &tmp_scm;
 	wait_for_unix_gc();
-	err = scm_send(sock, msg, siocb->scm);
+	err = scm_send(sock, msg, &scm);
 	if (err < 0)
 		return err;
 
@@ -1479,11 +1476,11 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	if (skb == NULL)
 		goto out;
 
-	err = unix_scm_to_skb(siocb->scm, skb, true);
+	err = unix_scm_to_skb(&scm, skb, true);
 	if (err < 0)
 		goto out_free;
 	max_level = err + 1;
-	unix_get_secdata(siocb->scm, skb);
+	unix_get_secdata(&scm, skb);
 
 	skb_put(skb, len - data_len);
 	skb->data_len = data_len;
@@ -1578,7 +1575,7 @@ restart:
 	unix_state_unlock(other);
 	other->sk_data_ready(other, len);
 	sock_put(other);
-	scm_destroy(siocb->scm);
+	scm_destroy(&scm);
 	return len;
 
 out_unlock:
@@ -1588,7 +1585,7 @@ out_free:
 out:
 	if (other)
 		sock_put(other);
-	scm_destroy(siocb->scm);
+	scm_destroy(&scm);
 	return err;
 }
 
@@ -1596,20 +1593,17 @@ out:
 static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 			       struct msghdr *msg, size_t len)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(kiocb);
 	struct sock *sk = sock->sk;
 	struct sock *other = NULL;
 	int err, size;
 	struct sk_buff *skb;
 	int sent = 0;
-	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	bool fds_sent = false;
 	int max_level;
 
-	if (NULL == siocb->scm)
-		siocb->scm = &tmp_scm;
 	wait_for_unix_gc();
-	err = scm_send(sock, msg, siocb->scm);
+	err = scm_send(sock, msg, &scm);
 	if (err < 0)
 		return err;
 
@@ -1666,7 +1660,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 
 
 		/* Only send the fds in the first buffer */
-		err = unix_scm_to_skb(siocb->scm, skb, !fds_sent);
+		err = unix_scm_to_skb(&scm, skb, !fds_sent);
 		if (err < 0) {
 			kfree_skb(skb);
 			goto out_err;
@@ -1695,8 +1689,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		sent += size;
 	}
 
-	scm_destroy(siocb->scm);
-	siocb->scm = NULL;
+	scm_destroy(&scm);
 
 	return sent;
 
@@ -1708,8 +1701,7 @@ pipe_err:
 		send_sig(SIGPIPE, current, 0);
 	err = -EPIPE;
 out_err:
-	scm_destroy(siocb->scm);
-	siocb->scm = NULL;
+	scm_destroy(&scm);
 	return sent ? : err;
 }
 
@@ -1760,7 +1752,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 			      int flags)
 {
 	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
-	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
 	int noblock = flags & MSG_DONTWAIT;
@@ -1811,16 +1803,12 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 	if (sock_flag(sk, SOCK_RCVTSTAMP))
 		__sock_recv_timestamp(msg, sk, skb);
 
-	if (!siocb->scm) {
-		siocb->scm = &tmp_scm;
-		memset(&tmp_scm, 0, sizeof(tmp_scm));
-	}
-	siocb->scm->creds = UNIXCB(skb).creds;
-	unix_set_secdata(siocb->scm, skb);
+	scm.creds = UNIXCB(skb).creds;
+	unix_set_secdata(&scm, skb);
 
 	if (!(flags & MSG_PEEK)) {
 		if (UNIXCB(skb).fp)
-			unix_detach_fds(siocb->scm, skb);
+			unix_detach_fds(&scm, skb);
 
 		sk_peek_offset_bwd(sk, skb->len);
 	} else {
@@ -1840,11 +1828,11 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 		sk_peek_offset_fwd(sk, size);
 
 		if (UNIXCB(skb).fp)
-			siocb->scm->fp = scm_fp_dup(UNIXCB(skb).fp);
+			scm.fp = scm_fp_dup(UNIXCB(skb).fp);
 	}
 	err = (flags & MSG_TRUNC) ? skb->len - skip : size;
 
-	scm_recv(sock, msg, siocb->scm, flags);
+	scm_recv(sock, msg, &scm, flags);
 
 out_free:
 	skb_free_datagram(sk, skb);
@@ -1892,8 +1880,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 			       struct msghdr *msg, size_t size,
 			       int flags)
 {
-	struct sock_iocb *siocb = kiocb_to_siocb(iocb);
-	struct scm_cookie tmp_scm = SCM_COOKIE_INIT;
+	struct scm_cookie scm = SCM_COOKIE_INIT;
 	struct sock *sk = sock->sk;
 	struct unix_sock *u = unix_sk(sk);
 	struct sockaddr_un *sunaddr = msg->msg_name;
@@ -1921,11 +1908,6 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 	 * while sleeps in memcpy_tomsg
 	 */
 
-	if (!siocb->scm) {
-		siocb->scm = &tmp_scm;
-		memset(&tmp_scm, 0, sizeof(tmp_scm));
-	}
-
 	err = mutex_lock_interruptible(&u->readlock);
 	if (err) {
 		err = sock_intr_errno(timeo);
@@ -1987,11 +1969,11 @@ again:
 		if (check_creds) {
 			/* Never glue messages from different writers */
 			if (!scm_creds_equal(&UNIXCB(skb).creds,
-			                     &siocb->scm->creds));
+			                     &scm.creds));
 				break;
 		} else {
 			/* Copy credentials */
-			siocb->scm->creds = UNIXCB(skb).creds;
+			scm.creds = UNIXCB(skb).creds;
 			check_creds = 1;
 		}
 
@@ -2017,7 +1999,7 @@ again:
 			sk_peek_offset_bwd(sk, chunk);
 
 			if (UNIXCB(skb).fp)
-				unix_detach_fds(siocb->scm, skb);
+				unix_detach_fds(&scm, skb);
 
 			if (skb->len)
 				break;
@@ -2025,13 +2007,13 @@ again:
 			skb_unlink(skb, &sk->sk_receive_queue);
 			consume_skb(skb);
 
-			if (siocb->scm->fp)
+			if (scm.fp)
 				break;
 		} else {
 			/* It is questionable, see note in unix_dgram_recvmsg.
 			 */
 			if (UNIXCB(skb).fp)
-				siocb->scm->fp = scm_fp_dup(UNIXCB(skb).fp);
+				scm.fp = scm_fp_dup(UNIXCB(skb).fp);
 
 			sk_peek_offset_fwd(sk, chunk);
 
@@ -2040,7 +2022,7 @@ again:
 	} while (size);
 
 	mutex_unlock(&u->readlock);
-	scm_recv(sock, msg, siocb->scm, flags);
+	scm_recv(sock, msg, &scm, flags);
 out:
 	return copied ? : err;
 }
-- 
1.8.1.4