block: fix ioprio_get(IOPRIO_WHO_PGRP) vs setuid(2)

stable inclusion
from stable-v5.10.85
commit 5dfe61147442cb9eedeb282630abf304144bc71c
bugzilla: 186032 https://gitee.com/openeuler/kernel/issues/I4QVI4

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5dfe61147442cb9eedeb282630abf304144bc71c

--------------------------------

commit e6a59aac 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: d69b78ba (ioprio: grab rcu_read_lock in sys_ioprio_{set,get}())
Acked-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NDavidlohr Bueso <dbueso@suse.de>
Link: https://lore.kernel.org/r/20211210182058.43417-1-dave@stgolabs.netSigned-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

block/ioprio.c

@@ -214,6 +214,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 				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)
@@ -223,6 +224,8 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 				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);