userns: Convert net/core/scm.c to use kuids and kgids

With the existence of kuid_t and kgid_t we can take this further
and remove the usage of struct cred altogether, ensuring we
don't get cache line misses from reference counts. For now
however start simply and do a straight forward conversion
I can be certain is correct.

In cred_to_ucred use from_kuid_munged and from_kgid_munged
as these values are going directly to userspace and we want to use
the userspace safe values not -1 when reporting a value that does not
map. The earlier conversion that used from_kuid was buggy in that
respect. Oops.

Cc: Eric Dumazet <[e-mail]>
Acked-by: David S. Miller <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

net/core/scm.c

···
 static __inline__ int scm_check_creds(struct ucred *creds)
 {
 	const struct cred *cred = current_cred();
+	kuid_t uid = make_kuid(cred->user_ns, creds->uid);
+	kgid_t gid = make_kgid(cred->user_ns, creds->gid);
+
+	if (!uid_valid(uid) || !gid_valid(gid))
+		return -EINVAL;
 
 	if ((creds->pid == task_tgid_vnr(current) || capable(CAP_SYS_ADMIN)) &&
-	    ((creds->uid == cred->uid   || creds->uid == cred->euid ||
-	      creds->uid == cred->suid) || capable(CAP_SETUID)) &&
-	    ((creds->gid == cred->gid   || creds->gid == cred->egid ||
-	      creds->gid == cred->sgid) || capable(CAP_SETGID))) {
+	    ((uid_eq(uid, cred->uid)   || uid_eq(uid, cred->euid) ||
+	      uid_eq(uid, cred->suid)) || capable(CAP_SETUID)) &&
+	    ((gid_eq(gid, cred->gid)   || gid_eq(gid, cred->egid) ||
+	      gid_eq(gid, cred->sgid)) || capable(CAP_SETGID))) {
 	       return 0;
 	}
 	return -EPERM;
···
 				goto error;
 			break;
 		case SCM_CREDENTIALS:
+		{
+			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));
···
 				p->pid = pid;
 			}
 
+			err = -EINVAL;
+			uid = make_kuid(current_user_ns(), p->creds.uid);
+			gid = make_kgid(current_user_ns(), p->creds.gid);
+			if (!uid_valid(uid) || !gid_valid(gid))
+				goto error;
+
 			if (!p->cred ||
-			    (p->cred->euid != p->creds.uid) ||
-			    (p->cred->egid != p->creds.gid)) {
+			    !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 = p->creds.uid;
-				cred->gid = cred->egid = p->creds.gid;
+				cred->uid = cred->euid = uid;
+				cred->gid = cred->egid = gid;
 				if (p->cred)
 					put_cred(p->cred);
 				p->cred = cred;
 			}
 			break;
+		}
 		default:
 			goto error;
 		}

net/core/sock.c

···
 	if (cred) {
 		struct user_namespace *current_ns = current_user_ns();
 
-		ucred->uid = from_kuid(current_ns, cred->euid);
-		ucred->gid = from_kgid(current_ns, cred->egid);
+		ucred->uid = from_kuid_munged(current_ns, cred->euid);
+		ucred->gid = from_kgid_munged(current_ns, cred->egid);
 	}
 }
 EXPORT_SYMBOL_GPL(cred_to_ucred);