userns: unshare_userns(&cred) should not populate cred on failure

commit 6160968cee8b90a5dd95318d716e31d7775c4ef3 upstream.

unshare_userns(new_cred) does *new_cred = prepare_creds() before
create_user_ns() which can fail. However, the caller expects that
it doesn't need to take care of new_cred if unshare_userns() fails.

We could change the single caller, sys_unshare(), but I think it
would be more clean to avoid the side effects on failure, so with
this patch unshare_userns() does put_cred() itself and initializes
*new_cred only if create_user_ns() succeeeds.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index d8c30db06c5b..6e50a44610ee 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -105,16 +105,21 @@ int create_user_ns(struct cred *new)
 int unshare_userns(unsigned long unshare_flags, struct cred **new_cred)
 {
         struct cred *cred;
+        int err = -ENOMEM;
 
         if (!(unshare_flags & CLONE_NEWUSER))
                 return 0;
 
         cred = prepare_creds();
-        if (!cred)
-                return -ENOMEM;
+        if (cred) {
+                err = create_user_ns(cred);
+                if (err)
+                        put_cred(cred);
+                else
+                        *new_cred = cred;
+        }
 
-        *new_cred = cred;
-        return create_user_ns(cred);
+        return err;
 }
 
 void free_user_ns(struct user_namespace *ns)