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>

2 files changed, 21 insertions, 5 deletions

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index c9fdde2bc73f..df73cf4b0290 100644
--- a/include/linux/netlink.h
+++ b/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;
diff --git a/include/net/scm.h b/include/net/scm.h
index 7dc0854f0b38..456695f5cbc4 100644
--- a/include/net/scm.h
+++ b/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);