writeback: Fix use after free in inode_switch_wbs_work_fn()

In the Linux kernel, the following vulnerability has been resolved:

writeback: Fix use after free in inode_switch_wbs_work_fn()

inode_switch_wbs_work_fn() has a loop like:

wb_get(new_wb);

while (1) {

list = llist_del_all(&new_wb->switch_wbs_ctxs);

/* Nothing to do? */

if (!list)

break;

... process the items ...

}

Now adding of items to the list looks like:

wb_queue_isw()

if (llist_add(&isw->list, &wb->switch_wbs_ctxs))

queue_work(isw_wq, &wb->switch_work);

Because inode_switch_wbs_work_fn() loops when processing isw items, it

can happen that wb->switch_work is pending while wb->switch_wbs_ctxs is

empty. This is a problem because in that case wb can get freed (no isw

items -> no wb reference) while the work is still pending causing

use-after-free issues.

We cannot just fix this by cancelling work when freeing wb because that

could still trigger problematic 0 -> 1 transitions on wb refcount due to

wb_get() in inode_switch_wbs_work_fn(). It could be all handled with

more careful code but that seems unnecessarily complex so let's avoid

that until it is proven that the looping actually brings practical

benefit. Just remove the loop from inode_switch_wbs_work_fn() instead.

That way when wb_queue_isw() queues work, we are guaranteed we have

added the first item to wb->switch_wbs_ctxs and nobody is going to

remove it (and drop the wb reference it holds) until the queued work

runs.