scm: Don't use struct ucred in NETLINK_CB and struct scm_cookie.

Passing uids and gids on NETLINK_CB from a process in one user
namespace to a process in another user namespace can result in the
wrong uid or gid being presented to userspace.  Avoid that problem by
passing kuids and kgids instead.

- define struct scm_creds for use in scm_cookie and netlink_skb_parms
  that holds uid and gid information in kuid_t and kgid_t.

- Modify scm_set_cred to fill out scm_creds by heand instead of using
  cred_to_ucred to fill out struct ucred.  This conversion ensures
  userspace does not get incorrect uid or gid values to look at.

- Modify scm_recv to convert from struct scm_creds to struct ucred
  before copying credential values to userspace.

- Modify __scm_send to populate struct scm_creds on in the scm_cookie,
  instead of just copying struct ucred from userspace.

- Modify netlink_sendmsg to copy scm_creds instead of struct ucred
  into the NETLINK_CB.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -153,6 +153,7 @@ struct nlattr {
 
 #include <linux/capability.h>
 #include <linux/skbuff.h>
+#include <net/scm.h>
 
 struct net;
 
@@ -162,7 +163,7 @@ static inline struct nlmsghdr *nlmsg_hdr(const struct sk_buff *skb)
 }
 
 struct netlink_skb_parms {
-	struct ucred		creds;		/* Skb credentials	*/
+	struct scm_creds	creds;		/* Skb credentials	*/
 	__u32			pid;
 	__u32			dst_group;
 	struct sock		*ssk;

include/net/scm.h

@@ -12,6 +12,12 @@
  */
 #define SCM_MAX_FD	253
 
+struct scm_creds {
+	u32	pid;
+	kuid_t	uid;
+	kgid_t	gid;
+};
+
 struct scm_fp_list {
 	short			count;
 	short			max;
@@ -22,7 +28,7 @@ struct scm_cookie {
 	struct pid		*pid;		/* Skb credentials */
 	const struct cred	*cred;
 	struct scm_fp_list	*fp;		/* Passed files		*/
-	struct ucred		creds;		/* Skb credentials	*/
+	struct scm_creds	creds;		/* Skb credentials	*/
 #ifdef CONFIG_SECURITY_NETWORK
 	u32			secid;		/* Passed security ID 	*/
 #endif
@@ -49,7 +55,9 @@ static __inline__ void scm_set_cred(struct scm_cookie *scm,
 {
 	scm->pid  = get_pid(pid);
 	scm->cred = cred ? get_cred(cred) : NULL;
-	cred_to_ucred(pid, cred, &scm->creds);
+	scm->creds.pid = pid_vnr(pid);
+	scm->creds.uid = cred ? cred->euid : INVALID_UID;
+	scm->creds.gid = cred ? cred->egid : INVALID_GID;
 }
 
 static __inline__ void scm_destroy_cred(struct scm_cookie *scm)
@@ -112,8 +120,15 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
 		return;
 	}
 
-	if (test_bit(SOCK_PASSCRED, &sock->flags))
-		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(scm->creds), &scm->creds);
+	if (test_bit(SOCK_PASSCRED, &sock->flags)) {
+		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),
+		};
+		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(ucreds), &ucreds);
+	}
 
 	scm_destroy_cred(scm);
 

net/core/scm.c

@@ -155,19 +155,21 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
 			break;
 		case SCM_CREDENTIALS:
 		{
+			struct ucred creds;
 			kuid_t uid;
 			kgid_t gid;
 			if (cmsg->cmsg_len != CMSG_LEN(sizeof(struct ucred)))
 				goto error;
-			memcpy(&p->creds, CMSG_DATA(cmsg), sizeof(struct ucred));
-			err = scm_check_creds(&p->creds);
+			memcpy(&creds, CMSG_DATA(cmsg), sizeof(struct ucred));
+			err = scm_check_creds(&creds);
 			if (err)
 				goto error;
 
-			if (!p->pid || pid_vnr(p->pid) != p->creds.pid) {
+			p->creds.pid = creds.pid;
+			if (!p->pid || pid_vnr(p->pid) != creds.pid) {
 				struct pid *pid;
 				err = -ESRCH;
-				pid = find_get_pid(p->creds.pid);
+				pid = find_get_pid(creds.pid);
 				if (!pid)
 					goto error;
 				put_pid(p->pid);
@@ -175,11 +177,14 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
 			}
 
 			err = -EINVAL;
-			uid = make_kuid(current_user_ns(), p->creds.uid);
-			gid = make_kgid(current_user_ns(), p->creds.gid);
+			uid = make_kuid(current_user_ns(), creds.uid);
+			gid = make_kgid(current_user_ns(), creds.gid);
 			if (!uid_valid(uid) || !gid_valid(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)) {

net/netlink/af_netlink.c

@@ -1399,7 +1399,7 @@ static int netlink_sendmsg(struct kiocb *kiocb, struct socket *sock,
 
 	NETLINK_CB(skb).pid	= nlk->pid;
 	NETLINK_CB(skb).dst_group = dst_group;
-	memcpy(NETLINK_CREDS(skb), &siocb->scm->creds, sizeof(struct ucred));
+	NETLINK_CB(skb).creds	= siocb->scm->creds;
 
 	err = -EFAULT;
 	if (memcpy_fromiovec(skb_put(skb, len), msg->msg_iov, len)) {