[PATCH] pidfd: Stop taking cred_guard_mutex

[ebiederm@xmission.com (Eric W. Biederman)]

During exec some file descriptors are closed and the files struct is
unshared.  But all of that can happen at other times and it has the
same protections during exec as at ordinary times.  So stop taking the
cred_guard_mutex as it is useless.

Furthermore he cred_guard_mutex is a bad idea because it is deadlock
prone, as it is held in serveral while waiting possibly indefinitely
for userspace to do something.

Cc: Sargun Dhillon <sargun@sargun.me>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/pid.c | 6 ------
 1 file changed, 6 deletions(-)

Christian if you don't have any objections I will take this one through
my tree.

I tried to figure out why this code path takes the cred_guard_mutex and
the archive on lore.kernel.org was not helpful in finding that part of
the conversation.

diff --git a/kernel/pid.c b/kernel/pid.c
index 60820e72634c..53646d5616d2 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -577,17 +577,11 @@ static struct file *__pidfd_fget(struct task_struct *task, int fd)
 	struct file *file;
 	int ret;
 
-	ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
-	if (ret)
-		return ERR_PTR(ret);
-
 	if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
 		file = fget_task(task, fd);
 	else
 		file = ERR_PTR(-EPERM);
 
-	mutex_unlock(&task->signal->cred_guard_mutex);
-
 	return file ?: ERR_PTR(-EBADF);
 }
 
-- 
2.20.1