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);