[PATCH 2/2] block: fix ioprio_get/set(IOPRIO_WHO_PGRP) vs setuid(2)

[Davidlohr Bueso]

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/set to take
the tasklist_lock in this case.

Fixes: d69b78ba1de (ioprio: grab rcu_read_lock in sys_ioprio_{set,get}())
Cc: Greg Thelen <gthe...@google.com>
Signed-off-by: Davidlohr Bueso <dbu...@suse.de>
---
 block/ioprio.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/ioprio.c b/block/ioprio.c
index 77bcab11dce5..4ede2da961bb 100644
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -119,11 +119,13 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, 
ioprio)
                                pgrp = task_pgrp(current);
                        else
                                pgrp = find_vpid(who);
+                       read_lock(&tasklist_lock);
                        do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
                                ret = set_task_ioprio(p, ioprio);
                                if (ret)
                                        break;
                        } while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
+                       read_unlock(&tasklist_lock);
                        break;
                case IOPRIO_WHO_USER:
                        uid = make_kuid(current_user_ns(), who);
@@ -207,6 +209,7 @@ 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)
@@ -216,6 +219,7 @@ 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);
-- 
2.26.2