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

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@redhat.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Link: https://lore.kernel.org/r/20211210182058.43417-1-dave@stgolabs.net
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/block/ioprio.c b/block/ioprio.c
index 313c14a..6f01d35 100644 (file)
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -220,6 +220,7 @@ 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)
@@ -229,6 +230,8 @@ 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);