The cred_guard_mutex is problematic.  The cred_guard_mutex is held
over the userspace accesses as the arguments from userspace are read.
The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other
threads are killed.  The cred_guard_mutex is held over
"put_user(0, tsk->clear_child_tid)" in exit_mm().

Any of those can result in deadlock, as the cred_guard_mutex is held
over a possible indefinite userspace waits for userspace.

Add exec_update_mutex that is only held over exec updating process
with the new contents of exec, so that code that needs not to be
confused by exec changing the mm and the cred in ways that can not
happen during ordinary execution of a process.

The plan is to switch the users of cred_guard_mutex to
exec_udpate_mutex one by one.  This lets us move forward while still
being careful and not introducing any regressions.

Link: https://lore.kernel.org/lkml/20160921152946.GA24210@dhcp22.suse.cz/ (local)
Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/ (local)
Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/ (local)
Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/ (local)
Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/ (local)
Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")
Signed-off-by: "Eric W. Biederman" <redacted>
---
 fs/exec.c                    | 9 +++++++++
 include/linux/sched/signal.h | 9 ++++++++-
 init/init_task.c             | 1 +
 kernel/fork.c                | 1 +
 4 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index d820a7272a76..ffeebb1f167b 100644
--- a/fs/exec.c
+++ b/fs/exec.c

@@ -1014,6 +1014,7 @@ static int exec_mmap(struct mm_struct *mm)
 {
 	struct task_struct *tsk;
 	struct mm_struct *old_mm, *active_mm;
+	int ret;
 
 	/* Notify parent that we're no longer interested in the old VM */
 	tsk = current;

@@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm)
 			return -EINTR;
 		}
 	}
+
+	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
+	if (ret)
+		return ret;