cred: remove task_is_dead() from __task_cred() validation

Commit 8f92054e7ca1 ("CRED: Fix __task_cred()'s lockdep check and banner
comment"):

    add the following validation condition:

        task->exit_state >= 0

    to permit the access if the target task is dead and therefore
    unable to change its own credentials.

OK, but afaics currently this can only help wait_task_zombie() which calls
__task_cred() without rcu lock.

Remove this validation and change wait_task_zombie() to use task_uid()
instead.  This means we do rcu_read_lock() only to shut up the lockdep,
but we already do the same in, say, wait_task_stopped().

task_is_dead() should die, task->exit_state != 0 means that this task has
passed exit_notify(), only do_wait-like code paths should use this.

Unfortunately, we can't kill task_is_dead() right now, it has already
acquired buggy users in drivers/staging.  The fix already exists.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 910a071..3281493 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1214,7 +1214,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
 	unsigned long state;
 	int retval, status, traced;
 	pid_t pid = task_pid_vnr(p);
-	uid_t uid = from_kuid_munged(current_user_ns(), __task_cred(p)->uid);
+	uid_t uid = from_kuid_munged(current_user_ns(), task_uid(p));
 	struct siginfo __user *infop;
 
 	if (!likely(wo->wo_flags & WEXITED))