userns,pidns: Force thread group sharing, not signal handler sharing.

The code that places signals in signal queues computes the uids, gids,
and pids at the time the signals are enqueued.  Which means that tasks
that share signal queues must be in the same pid and user namespaces.

Sharing signal handlers is fine, but bizarre.

So make the code in fork and userns_install clearer by only testing
for what is functionally necessary.

Also update the comment in unshare about unsharing a user namespace to
be a little more explicit and make a little more sense.

Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 4109f8320684..f65a0a06a8c0 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -976,8 +976,8 @@ static int userns_install(struct nsproxy *nsproxy, struct ns_common *ns)
         if (user_ns == current_user_ns())
                 return -EINVAL;
 
-        /* Threaded processes may not enter a different user namespace */
-        if (atomic_read(&current->mm->mm_users) > 1)
+        /* Tasks that share a thread group must share a user namespace */
+        if (!thread_group_empty(current))
                 return -EINVAL;
 
         if (current->fs->users != 1)