[PATCH 4.9 23/42] block: fix ioprio_get(IOPRIO_WHO_PGRP) vs setuid(2)

[Greg Kroah-Hartman]

From: Davidlohr Bueso <dave@xxxxxxxxxxxx>

commit e6a59aac8a8713f335a37d762db0dbe80e7f6d38 upstream.

do_each_pid_thread(PIDTYPE_PGID) can race with a concurrent
change_pid(PIDTYPE_PGID) that can move the task from one hlist
to another while iterating. Serialize ioprio_get to take
the tasklist_lock in this case, just like it's set counterpart.

Fixes: d69b78ba1de (ioprio: grab rcu_read_lock in sys_ioprio_{set,get}())
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
Link: https://lore.kernel.org/r/20211210182058.43417-1-dave@xxxxxxxxxxxx
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 block/ioprio.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -202,6 +202,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which,
 				pgrp = task_pgrp(current);
 			else
 				pgrp = find_vpid(who);
+			read_lock(&tasklist_lock);
 			do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
 				tmpio = get_task_ioprio(p);
 				if (tmpio < 0)
@@ -211,6 +212,8 @@ SYSCALL_DEFINE2(ioprio_get, int, which,
 				else
 					ret = ioprio_best(ret, tmpio);
 			} while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
+			read_unlock(&tasklist_lock);
+
 			break;
 		case IOPRIO_WHO_USER:
 			uid = make_kuid(current_user_ns(), who);