sched/rt: pick_next_rt_entity(): check list_entry

commit 7c4a5b89a0b5a57a64b601775b296abf77a9fe97 upstream.

Commit 326587b8 ("sched: fix goto retry in pick_next_task_rt()")
removed any path which could make pick_next_rt_entity() return NULL.
However, BUG_ON(!rt_se) in _pick_next_task_rt() (the only caller of
pick_next_rt_entity()) still checks the error condition, which can
never happen, since list_entry() never returns NULL.
Remove the BUG_ON check, and instead emit a warning in the only
possible error condition here: the queue being empty which should
never happen.

Fixes: 326587b8 ("sched: fix goto retry in pick_next_task_rt()")
Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Phil Auld <pauld@redhat.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v3-1-b1a71bd1ac6b@diag.uniroma1.it


Signed-off-by: Sasha Levin <sashal@kernel.org>
[ Fixes CVE-2023-1077: sched/rt: pick_next_rt_entity(): check list_entry
  An insufficient list empty checking in pick_next_rt_entity().  The
  _pick_next_task_rt() checks pick_next_rt_entity() returns NULL or not
  but pick_next_rt_entity() never returns NULL.  So, even if the list is
  empty, _pick_next_task_rt() continues its process. ]
Signed-off-by: Srish Srinivasan <ssrish@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>