net: fix incorrect credentials passing

commit 83f1b4ba upstream.

Commit 257b5358 ("scm: Capture the full credentials of the scm
sender") changed the credentials passing code to pass in the effective
uid/gid instead of the real uid/gid.

Obviously this doesn't matter most of the time (since normally they are
the same), but it results in differences for suid binaries when the wrong
uid/gid ends up being used.

This just undoes that (presumably unintentional) part of the commit.
Reported-by: Andy Lutomirski <luto@amacapital.net>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Serge E. Hallyn <serge@hallyn.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: scm_set_cred() does user namespace conversion
 of euid/egid using cred_to_ucred().  Add and use cred_real_to_ucred() to
 do the same thing for real uid/gid.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

include/linux/socket.h

@@ -317,6 +317,7 @@ struct ucred {
 #define IPX_TYPE	1
 
 extern void cred_to_ucred(struct pid *pid, const struct cred *cred, struct ucred *ucred);
+extern void cred_real_to_ucred(struct pid *pid, const struct cred *cred, struct ucred *ucred);
 
 extern int memcpy_fromiovec(unsigned char *kdata, struct iovec *iov, int len);
 extern int memcpy_fromiovecend(unsigned char *kdata, const struct iovec *iov,

include/net/scm.h

@@ -50,7 +50,7 @@ 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);
+	cred_real_to_ucred(pid, cred, &scm->creds);
 }
 
 static __inline__ void scm_destroy_cred(struct scm_cookie *scm)

net/core/sock.c

@@ -761,6 +761,20 @@ void cred_to_ucred(struct pid *pid, const struct cred *cred,
 }
 EXPORT_SYMBOL_GPL(cred_to_ucred);
 
+void cred_real_to_ucred(struct pid *pid, const struct cred *cred,
+			struct ucred *ucred)
+{
+	ucred->pid = pid_vnr(pid);
+	ucred->uid = ucred->gid = -1;
+	if (cred) {
+		struct user_namespace *current_ns = current_user_ns();
+
+		ucred->uid = user_ns_map_uid(current_ns, cred, cred->uid);
+		ucred->gid = user_ns_map_gid(current_ns, cred, cred->gid);
+	}
+}
+EXPORT_SYMBOL_GPL(cred_real_to_ucred);
+
 int sock_getsockopt(struct socket *sock, int level, int optname,
 		    char __user *optval, int __user *optlen)
 {